IMMUNOBIOLOGICAL SURVEILLANCE SYSTEM
Biological significance immunobiological surveillance system IBN consists of control (supervision) over the individual and homogeneous cellular and molecular composition of the body.
The detection of a carrier of foreign genetic or antigenic information (molecules, viruses, cells or their fragments) is accompanied by its inactivation, destruction and, as a rule, elimination. At the same time, cells of the immune system are able to retain a “memory” of this agent.
Repeated contact of such an agent with the cells of the IBN system causes the development of an effective response, which is formed with the participation of both specific immune defense mechanisms and nonspecific resistance factors of the body (Fig. 1).
Rice. 1. Structure of the body’s immunobiological surveillance system. NK - natural killers (natural killers). A cells are antigen presenting cells.
The main ideas in the system about the mechanisms of surveillance of the individual and homogeneous antigenic composition of the body include the concepts of Ag, immunity, the immune system and the system of nonspecific defense factors of the body.
Antigens
The initial link in the process of forming an immune response is the recognition of a foreign agent - antigen (Ag). The origin of this term is associated with the period of searching for agents, substances or “bodies” that neutralize the factors that cause the disease, and specifically we were talking about the toxin of the diphtheria bacillus. These substances were first called “antitoxins”, and soon the more general term “antibody” was introduced. The factor leading to the formation of an “antibody” was designated as an “antigen”.
Antigen- a substance of exo- or endogenous origin that causes the development of immune reactions (humoral and cellular immune responses, delayed-type hypersensitivity reactions and the formation of immunological memory).
Considering the ability of Ags to induce tolerance, an immune or allergic response, they are also called, respectively, tolerogens, immunogens or allergens, respectively.
The different results of the interaction between Ag and the body (immunity, allergy, tolerance) depend on a number of factors: on the properties of the Ag itself, the conditions of its interaction with the immune system, the state of reactivity of the body, and others (Fig. 2).
Rice. 2. Potential effects of the antigen in the body.
Antigenic determinant
The formation of Ab and sensitization of lymphocytes is not caused by the entire Ag molecule, but only by a special part of it - the antigenic determinant, or epitope. In most protein Ags, such a determinant is formed by a sequence of 4–8 amino acid residues, and in polysaccharide Ags - 3–6 hexose residues. The number of determinants for one Ag may be different. Thus, egg albumin has at least 5 of them, diphtheria toxin has at least 80, and thyroglobulin has more than 40.
Types of antigens
In accordance with the structure and origin, Ag is divided into several types.
Depending on the structure, protein and non-protein Ags are distinguished.
1). Proteins or complex substances (glycoproteins, nucleoproteins, lipids). Their molecules may have several different antigenic determinants;
2). Substances that do not contain protein are called haptens. These include many mono-, oligo- and polysaccharides, lipids, glycolipids, artificial polymers, inorganic substances(compounds of iodine, bromine, bismuth), some drugs. Haptens themselves are non-immunogenic. However, after they are attached (usually covalently) to a carrier - a protein molecule or protein ligands cell membranes- they acquire the ability to cause an immune response. A hapten molecule usually contains only one antigenic determinant.
Depending on the origin, exogenous and endogenous Ag are distinguished.
1. Exogenous Ag divided into infectious and non-infectious.
b) Non-infectious (foreign proteins; protein-containing compounds; Ag and haptens in dust, food products, pollen, a number of drugs).
2. Endogenous Ag(autoantigens) appear when proteins and protein-containing molecules of one’s own cells, non-cellular structures and body fluids are damaged, when haptens are conjugated with them, as a result of mutations leading to the synthesis of abnormal proteins, and when the immune system malfunctions. In other words, in all cases when Ag is recognized as foreign.
Immunity
In immunology, the term “immunity” is used in three meanings.
2. To indicate the reactions of the IBN system against Ag.
3. To designate the physiological form of immunogenic reactivity of the body, observed when cells of the immune system come into contact with a genetically or antigenically foreign structure. As a result, this structure is subject to destruction and, as a rule, is eliminated from the body.
The immune system
The immune system- a complex of organs and tissues containing immunocompetent cells and ensuring the antigenic individuality and homogeneity of the body by detecting and, as a rule, destroying and eliminating foreign Ag from it. The immune system consists of central and peripheral organs.
To the central (primary) bodies include bone marrow and thymus gland. They undergo antigen-independent division and maturation of lymphocytes, which subsequently migrate to the peripheral organs of the immune system.
To peripheral (secondary) organs include the spleen, lymph nodes, tonsils, and lymphoid elements of a number of mucous membranes. In these organs, both antigen-independent and antigen-dependent proliferation and differentiation of lymphocytes occur. As a rule, mature lymphocytes first come into contact with Ag in peripheral lymphoid organs.
† The colonization of the peripheral organs of the immune system by T- and B-lymphocytes coming from the central organs of the immune system does not occur chaotically. Each population of lymphocytes migrates from blood vessels into certain lymphoid organs and even into their different regions. Thus, B-lymphocytes predominate in the spleen (in its red pulp, as well as along the periphery of the white) and Peyer’s patches of the intestine (in the centers of the follicles), and T-lymphocytes predominate in the lymph nodes (in the deep layers of their cortex and in the perifollicular space) .
† In organism healthy person in the process of lymphopoiesis, more than 10 9 varieties of homogeneous clones of lymphocytes are formed. Moreover, each clone expresses only one type of specific antigen-binding receptor. Most lymphocytes in peripheral organs of the immune system are not permanently attached to them. They constantly circulate with blood and lymph both between various lymphoid organs and in all other organs and tissues of the body. Such lymphocytes are called recirculating lymphocytes.
† Biological meaning of recycling of T- and B-lymphocytes:
‡ Firstly, the implementation of constant surveillance of the antigenic structures of the body.
‡ Secondly, the implementation of intercellular interactions (cooperation) of lymphocytes and mononuclear phagocytes, which is necessary for the development and regulation of immune reactions.
The specific part of an antigen or hapten that reacts with the immune system is called an antigenic determinant or epitope. It is usually a small part of the molecule and often consists of only a few (four to eight) amino acids or sugar residues. One antigenic molecule can carry several different epitopes, each with a characteristic, rigidly fixed configuration, which is determined by the primary, secondary or tertiary structure of the molecule. These different antigenic determinants are recognized separately by the immune system, and the antibodies that are synthesized react only with a single epitope (that is, they are specific).
Types of antigens
A. External antigens: antigens can be external, that is, enter the body from the outside; they include microorganisms, transplanted cells and foreign particles that can enter the body through nutritional, inhalation or parenteral routes.
B. Internal antigens: internal antigens arise from damaged molecules of the body (for example, when they are combined with a hapten, when their own molecules are partially denatured, or when cells are transformed during the formation of a tumor), which are recognized as “foreign”.
B. Hidden antigens: Certain antigens (for example, nervous tissue, lens proteins and sperm) are anatomically separated from the immune system by histo-hematological barriers early in embryogenesis; therefore, tolerance to these molecules does not arise and their entry into the bloodstream in the postnatal period can lead to an immune response. Immunological reactivity against altered or latent self-antigens occurs in some autoimmune diseases.
Antigen recognition
For an immune response to develop, external antigens must first be recognized by the immune system. The recognition mechanisms have not been sufficiently studied; they depend on the nature (type) of the antigen, the route of its penetration into the body, etc. The optimal immune response to the largest number of antigens occurs only after the interaction of the antigen with macrophages, T- and B-lymphocytes (Fig. 10.1). The macrophage plays the role of a cell that “processes” the antigen. Dendritic reticular cells in lymphoid follicles and interdigitating reticular cells in the paracortical zone of lymph nodes are also thought to be specialized macrophages adapted to “process” antigens for B and T cells, respectively (see below).
“Treatment” means that the antigen absorbed by the macrophage is again brought to its surface in complex with the MHC molecule (Major Histocompatibility Complex).
Antigen receptors on T cells recognize the antigen-MHC molecule combination on the macrophage, resulting in T cell activation and the release of various lymphokines (Table 10.3). T-helpers recognize the antigen in complex with the MHC class II molecule, and T-suppressors - with the MHC class I molecule. The typical form of B cell activation (T cell-dependent) involves its interaction with both macrophages and T cells. B cells recognize some multivalent antigens directly (T cell-independent antigens).
CELLULAR BASIS OF THE IMMUNE RESPONSE
Lymphoid system
The immune response is carried out by the body's lymphoid system, which is divided into central and peripheral organs of immunogenesis.
Central organs of immunogenesis
TO central authorities Immunogenesis includes the thymus and bone marrow, in which initial, semi-stem lymphoid cells arise in the prenatal period (during this period, diversity and tolerance arise). It is believed that in humans the final development of diversity and tolerance will be completed within a few months after birth).
Peripheral organs of immunogenesis
Peripheral organs of immunogenesis include lymph nodes, the spleen, the Pirogov-Waldeyer ring (tonsils of the pharynx) and lymphatic follicles in the intestinal walls, in which mature lymphocytes that respond to antigenic stimulation accumulate.
Peripheral blood also contains lymphocytes. Circulating lymphocytes constitute a pool of cells that continuously exchange with cells of peripheral lymphoid tissue.
LYMPHOCYTES
Lymphocytes are formed in the embryonic period from the lymphoid germ in the bone marrow. Lymphocytes can be classified based on where they develop: 1) T lymphocytes (thymus-dependent) develop in the thymus and 2) B lymphocytes, which develop outside the thymus. B lymphocytes develop in birds in the bursa of Fabricius ( bursa- bag, hence the term “B-cells”); the functional equivalent in humans is the fetal liver or bone marrow.
Inactive small lymphocytes are cells approximately 8-10 microns in diameter, with a small volume of cytoplasm and a spherical nucleus that occupies almost the entire cell. The nucleus contains condensed chromatin, which appears distinctly basophilic in conventional staining. All inactive populations of lymphocytes are morphologically similar to each other and can only be differentiated by immunological and immunomorphological methods (Table 10.1).
T lymphocytes (T cells)
A. Distribution of T cells in the body: T lymphocytes arise in the embryonic thymus. In the postembryonic period, after maturation, T-lymphocytes settle in the T-zones of peripheral lymphoid tissue. These areas include:
Paracortical zone of lymph nodes and the space between lymphoid follicles (70% of lymphocytes in lymph nodes are T-lymphocytes);
Periarterial zones of lymphoid follicles in the white pulp of the spleen (40% of splenic lymphocytes are T cells).
T lymphocytes continuously and actively circulate between peripheral blood and peripheral lymphoid tissue. Between 80 and 90 percent of peripheral blood lymphocytes are T cells.
B. Transformation of T cells: After stimulation (activation) by a specific antigen, T lymphocytes transform into large, actively dividing cells called transformed T lymphocytes, or T immunoblasts, from which the executive T cells then arise. T-immunoblasts are 15-20 µm in diameter, with a large volume of cytoplasm and an irregular nucleus with light chromatin and nucleolus; the nucleus is located in the center of the cell. T-immunoblasts can be distinguished from B-immunoblasts only by immunomorphological methods. Effector T cells are morphologically similar to inactive small lymphocytes and are often called sensitized cells, cytotoxic cells, or killer T cells.
This process of T cell transformation constitutes the developmental (enhancement) stage of the immune response (Fig. 10.1), during which several T cells bearing receptors that recognize a given specific antigen form a large clone of executive T cells active against the same the antigen itself, because they have a corresponding receptor. The full process of T cell activation begins when macrophages intercept the antigen and, through some mechanism that is not yet well understood, “process” the antigen and reexport it to the cell surface in conjunction with MHC molecules before interacting with the T cell. Recognition occurs only when the T cell carries a specific receptor capable of recognizing the antigen-MHC molecule complex.
B. Functions of effector T cells: effector T cells play an important role in three functions of the immune system:
Cellular immunity;
Regulating the activity of B cells;
Delayed (IV) type hypersensitivity.
1. Cellular immunity: includes two main aspects:
- cytotoxic cells carrying surface antigens cause direct cell damage (cytotoxic or killer cells). Direct cytotoxicity occurs in immunological responses to antigens on the surface of neoplastic cells, transplanted tissues, and virus-infected cells. Cytotoxic T cells possibly cause lysis by forming pores in the cytoplasmic membranes of antigen-positive cells.
- Lymphokine production: Executive T cells play a critical role in shaping the immune response by producing soluble proteins (lymphokines) that regulate the functions of certain cells, such as macrophages and other lymphocytes (Table 10.3).
2. Regulation of B-lymphocyte activity: two important subtypes of T lymphocytes are involved in regulating B lymphocyte function.
Helper T cells (CD4 antigen-positive) help in the activation and transformation of B lymphocytes and in the synthesis of immunoglobulins. Suppressor T cells (CD8 antigen-positive) inhibit B cell activation and regulate the synthesis of immunoglobulins. Helper and suppressor T cells also exert similar regulatory influences on cellular immunity. However, a subtype of CD4-positive “helper” cells may have a purely suppressive effect by stimulating CD8-positive suppressor cells. The normal ratio of helper T cells to suppressor T cells (CD4/CD8 ratio) in peripheral blood is 0.9–2.7, with slight variations in the very young and very old. This ratio can be greatly reduced in certain diseases, including immunodeficiency disorders, delayed type IV hypersensitivity, and HIV infection.
D. Morphological identification of T-lymphocyte subpopulations: T lymphocytes and their subtypes are morphologically indistinguishable from each other or from B lymphocytes and are characterized by the presence of antigens that act as immunological markers. These antigens can be detected by specific monoclonal antibodies (Table 10.1). The use of these antibodies in the immunofluorescent or immunoperoxidase method also makes it possible to determine the localization of various T-subpopulations of lymphocytes in lymphoid tissue. Genetic techniques that detect rearrangements of T cell receptor genes also aid in T cell identification. Other methods, such as the E-rosette test, are becoming obsolete.
B lymphocytes
A. Distribution of B cells in the body: B lymphocytes develop in the functional equivalent of the avian bursa of Fabricius (probably in the embryonic bone marrow of mammals), undergoing a complex process that includes proliferation and division into classes. The B lymphocytes then spread through the bloodstream to the B regions of the peripheral lymphoid tissue. These areas include: 1) reactive (secondary or germinal) centers of follicles and sinuses of the medulla of lymph nodes (30% of lymphocytes in lymph nodes are B-cells); 2) reactive centers in the follicles of the white pulp of the spleen (40% of splenic lymphocytes are B-cells). The term "primary follicle" is used to refer to a collection of B cells in the lymph nodes or spleen that do not exhibit proliferative activity. Like T cells, B cells also constantly circulate between lymphoid tissue and peripheral blood, but less actively. B cells make up 10-20% percent of the total number of peripheral blood lymphocytes.
B. Transformation of B cells: After stimulation with a specific antigen, B lymphocytes transform into plasma cells. This process occurs in stages, with the formation of a number of intermediate forms that form the reactive (germinative) center of the follicle. Plasma cells synthesize immunoglobulins (antibodies) that are specific for the antigen. The formation of circulating antibodies specific for antigens is the basis of acquired immunity, called humoral immunity.
B. Morphological identification of B cells: Plasma cells are effector (executive) B cells. Plasmocytes have a characteristic morphological structure (Table 10.2). Plasmocytes have dimensions of 12-15 microns in diameter, basophilic cytoplasm (basophilia is explained by the presence of a large amount of RNA required for the synthesis of immunoglobulins), in which the Golgi zone is found, visible as a pale area located next to the nucleus, located eccentrically; Chromatin in the nucleus is located in the form of large clumps along the periphery (in the form of a “cart wheel” or “dial”). Immunoglobulins can be detected in the cytoplasm by immunological methods.
Other B lymphocytes can only be identified by immunological, immunomorphological and genetic methods. Immunofluorescence or immunoperoxidase methods, using antibodies to human immunoglobulin, detect the presence of surface immunoglobulin (on maturing B cells) and cytoplasmic immunoglobulin (in plasma cells). Specific monoclonal antibodies that react with B cells are also used (Table 10.1). Genetic techniques that detect the presence of rearranged immunoglobulin genes can also help identify B lymphocytes.
Null cells (NK cells and K cells)
Null cells are a heterogeneous group of lymphocytes that do not have the ability to form E-rosettes (an immunoassay previously used to identify T lymphocytes) and do not carry surface immunoglobulin (hence, unlabeled or “null” cells). This group includes some cells that are clearly T or B cells, as recently demonstrated by genetic and monoclonal antibody techniques, but the designation of these cells has been abandoned. The population of “null” cells represents T and B cells that are in the early stages of differentiation, before the appearance of a large number of markers on their surface. “Zero” cells make up 5-10% of all peripheral blood lymphocytes.
Some "null" cells have cytotoxic activity and are called natural killer (NK) cells; they can destroy some foreign cells, even if the body has never encountered this antigen. Others (called K cells) are involved in the destruction of cells by antibodies (antibody-dependent cell-mediated cytotoxicity (ADCC)).
There is evidence that the activities exhibited by NK cells and K cells are 2 various functions one type of cell. NK cells may play a protective role in the tumor process by eliminating potentially neoplastic cells.
MACROPHAGES (blood monocytes and tissue histiocytes)
A. Distribution in the body: Macrophages are different from lymphocytes, but also play an important role in the immune response, both as antigen-processing cells when the response occurs, and as phagocytes in the executive role. In the blood they are called monocytes; in tissues - histiocytes or tissue macrophages. Studies of hematopoiesis in the bone marrow of animals and humans have established that all macrophages arise from monocyte precursors in the bone marrow. Macrophages are found in all tissues of the body (histiocytes), as well as in the lymph nodes, where they are located both diffusely and fixedly in the subcapsular space and in the sinuses of the medulla. Tissue macrophages are also found in the red pulp sinuses of the spleen. In the liver, macrophages are known as Kupffer cells, in the lungs as alveolar macrophages, and in brain tissue as microglia. In peripheral blood and bone marrow they are detected in the form of monocytes and their precursors. Dendritic reticular cells in the follicles of the lymph nodes and interdigitating reticular cells in the paracortical zone are specialized antigen “processing” cells for B and T lymphocytes, respectively. Although their origin is unknown, they are assumed to be macrophages. In older literature, the term "reticuloendothelial system" was used to refer to these cell types.
B. Identification of macrophages: macrophages contain numerous cytoplasmic enzymes and can be identified in tissues by histochemical methods that detect these enzymes. Some enzymes, such as muramidase (lysozyme) and chymotrypsin, can be detected by a labeled antibody test (immunohistochemistry), which uses antibodies against the enzyme proteins. Such monoclonal antibodies against various CD antigens are widely used to identify macrophages (Table 10.1; CD11, CD68).
B. Functions of macrophages: Macrophage functions include phagocytosis, antigen processing, and interaction with cytokines.
1. Phagocytosis:
Non-immune phagocytosis: macrophages are able to phagocytose foreign particles, microorganisms and the remains of damaged cells directly, without inducing an immune response. However, phagocytosis of microorganisms and their destruction are greatly facilitated by the presence of specific immunoglobulins, complement and lymphokines, which are produced by immunologically activated T lymphocytes (Table 10.3).
Immune phagocytosis: macrophages have surface receptors for the C3b and Fc fragment of immunoglobulins. Any particles that are coated with immunoglobulin or complement (opsonized) are phagocytosed much more easily than “naked” particles.
2. “Processing” of antigens: macrophages “process” antigens and present them to B and T lymphocytes in the required form (Fig. 10.1); this cellular interaction involves the simultaneous recognition by lymphocytes of MHC molecules and “processed antigens” found on the surface of macrophages.
3. Interaction with cytokines: macrophages interact with cytokines produced by T lymphocytes (Table 10.3) to protect the body against certain damaging agents. A typical result of such interaction is the formation of granulomas. Macrophages also produce cytokines, including interleukin-1, interferon-β, and T- and B-cell growth factors (Table 10.3). Various interactions of lymphocytes and macrophages in tissues manifest themselves morphologically during chronic inflammation.
IMMUNOGLOBULINS (antibodies)
Synthesis of immunoglobulins: immunoglobulins are synthesized by plasma cells, which are formed from transformed, antigen-stimulated B-lymphocytes (B-immunoblasts). All immunoglobulin molecules synthesized by a single plasma cell are identical and have specific reactivity against a single antigenic determinant. Likewise, all plasma cells produced by transformation and proliferation of a single B lymphocyte precursor are identical; that is, they constitute a clone. Immunoglobulin molecules synthesized by cells of different plasma cell clones have different amino acid sequences, which determines the different tertiary structure of the molecules and gives different specificity to the antibody (that is, they react with different antigens). These differences in amino acid sequence occur in the so-called V (variable, variable) region of the immunoglobulin molecule (Fig. 10.3).
Structure of immunoglobulins(Figure 10.3): Most immunoglobulin molecules are composed of two heavy (H) chains and two light (L) chains linked by disulfide bonds. Light chains consist of either two k chains or two l chains. Heavy chains can be one of five classes (IgA, IgG, IgM, IgD, and IgE) (Table 10.4). There are several subclasses of heavy chains (isotypes). These different immunoglobulin chains are antigens to animals and have distinct antigenic determinants so that when administered to animals, the antibodies produced against them can be used to recognize and detect different types of light chains and classes of heavy chains in humans.
Each chain has a constant and a variable region. Constant the region remains constant in amino acid sequence and antigenicity within a given class of immunoglobulins; variable the region, on the contrary, is characterized by great variability in the amino acid sequence. It is in the variable part of the chain that the reaction of the compound with the antigen occurs. Each IgG molecule consists of two linked chains that form two antigen-binding sites (Fig. 10.3). The variable region of each chain contains hypervariable regions - three in the light chains and four in the heavy chains. The amino acid sequence variations in these hypervariable regions determine the specificity of the antibody. Under certain conditions, these hypervariable regions can also act as antigens (idiotypes). Antibody against idiotypes, i.e. produced against the hypervariable region of antibodies has a limited range of reactivity and binds only to immunoglobulin molecules that have this hypervariable region. In essence, antibody reactivity against idiotypes is limited exclusively to specific antibodies derived from a single clone. Although the above applies strictly to IgG, other classes of immunoglobulins have the same basic structure, except that IgM is a pentamer (that is, composed of 5 basic units (molecules) linked at the Fc termini) and IgA generally exists as dimer.
Permanent site Each immunoglobulin molecule has receptors for complement, and there is also a site on the Fc fragment that binds to cells that have Fc receptors (which is necessary for the implementation of cellular immunity). Inherited antigenic differences between heavy chains constitute allotypes. Immunoglobulin molecules can be broken down into pieces by various proteolytic enzymes. When exposed to papain, the molecule is divided in the region of heavy chain divergence (“forks”) (Fig. 10.3) into two Fab fragments and one Fc fragment (crystallizing). Pepsin breaks the molecule into an F(ab)'2 fragment and an Fc fragment. The Fc fragment is a permanent region; the lack of variability in the amino acid sequence is the main reason for the possibility of crystallization of this fragment. Fab and F(ab)'2 fragments carry one and two antigen-binding sites, respectively. The Fc fragment carries specific antigens, including those that define the immunological distinction of the five main classes of antibodies. The complement fixation site is also located on the Fc fragment. The enzymatic digestion method has historical meaning in the process of elucidating the structure of immunoglobulins.
Regulation of antibody production: Antibody production begins after B cells are activated by antigen. The maximum concentration of antibodies in the serum is observed from 1 to 2 weeks and then begins to decrease. The continued presence of free antigen maintains the response until increasing antibody levels lead to increased clearance of antigen and thus cessation of B cell stimulation. There are also more subtle mechanisms for regulating the synthesis of immunoglobulins. T helper cells (CD4 positive) play an important role in regulating the response of B cells to a large number of antigens and their constant presence increases the production of antibodies. This effect is due, at least in part, to the release of lymphokines (Table 10.3). T-suppressors (CD8-positive) have the opposite effect, causing a decrease in the immune response; strong response suppression may be one of the mechanisms underlying tolerance. One additional regulatory mechanism is the production of anti-idiotypes (i.e. antibodies against one's own antibodies (autoantibodies)). It is assumed that in an immune response, the production of a specific antibody is necessarily accompanied by the production of a second antibody (anti-idiotype) with specificity against the variable (V) sequences (idiotypes or antigen-binding regions) of the first antibody. An anti-idiotype antibody is capable of recognizing idiotypes on the B-cell antigen receptor (which is constructed from an immunoglobulin identical in structure to the idiotype of the first antibody), thus it competes with the antigen and serves to inhibit B-cell activation.
RECOGNITION OF ANTIGENS AND THE BASIS OF ANTIGENE RECEPTOR DIVERSITY
There are a large number of different antibodies. They all react with a huge number of different antigens. Likewise, a huge number of T cells recognize a huge variety of antigens. Specific antigen recognition is carried out by lymphocytes, which have receptors for the antigen on their surfaces. There are a huge number of receptors with varying specificities that react with the entire range of known antigens, but each lymphocyte has receptors for only a single antigen. It follows that there are a huge number of lymphocytes (approximately 106-109), each having one single type of receptor. The antigen receptors of B lymphocytes are immunoglobulins. The action of the gene rearrangement mechanism (see below) leads to the appearance of a variety of immunoglobulin molecules that serve as receptors for antigens on the cell surface and, ultimately, represent a specific immunoglobulin (antibody) that will be secreted by plasma cells after the immune response occurs. In a simplified form, an antigen selects lymphocytes that have receptors (i.e., B cell surface immunoglobulin) that match it (fit together like a key to a lock). This interaction leads to the division and transformation of the B cell, and ultimately to the formation of a clone of plasma cells that secrete antibody molecules with special binding sites that are essentially the same as those located on the cell surface of the original lymphocyte that recognized the antigen (Fig. 10.1). T cells also have receptors for antigens, and T cell populations have a similar degree of diversity. The T cell receptor consists of a pair of polypeptide chains (a and b chains), with each chain having a variable and a constant region, so the receptor is similar to the B cell receptor (which is a surface immunoglobulin). The T cell receptor can thus be regarded as a member of the “super immunoglobulin family,” which includes not only immunoglobulins but also other molecules involved in cell interaction and recognition, all of which have a common evolutionary origin. The diversity of T cell antigen recognition receptors is formed in the early embryonic period through a mechanism of gene rearrangement that is similar to the mechanism of formation of immunoglobulin diversity. Also, in parallel with the activation of B cells, the antigen selects T cells bearing receptors with the appropriate specificity, and thus stimulates the proliferation of a specific clone of T cells, which results in the formation of a generation of numerous effector T cells of identical specificity. Note that antigen recognition by T cells is a complex process involving spatial interaction of the antigen with the MHC molecule on macrophages and the T cell antigen receptor with the participation of CD3 and CD4 or CD8 molecules on T cells. Helper T cells recognize antigens bound to MHC class II molecules, and suppressor T cells and cytotoxic T cells recognize antigens bound to MHC class I molecules. T cells bearing a receptor composed of gamma and delta chains have been described, but their function is unknown.
THE EMERGENCE OF DIVERSITY: THE GENE “SHUGGING” MECHANISM
The diversity of antigen receptors on B and T cells arises at the DNA level during the differentiation of lymphoid progenitors in the embryonic period. The genes involved in this process are located on chromosomes 2 (k chain), 22 (l chain), 14 (heavy chains, a and g chains of T-cell receptors) and 7 (b and d chains of T-cell receptors). Although each of these genes functions as a "gene unit" for the production of a chain of polypeptides, each gene exists on the DNA chain as a complex "multigene" consisting of a large number of different DNA segments that can be folded or assembled together in various modifications, resulting in many different DNA patterns. For example, a heavy chain multigene contains up to 200 different V (variable) segments (VH); each encoding corresponds to a specific amino acid sequence in the antigen-binding region (variable region) of the immunoglobulin heavy chain. The heavy chain gene also contains multiple D (diversity), J (joining) and C (constant) segments, one for each subclass and class of heavy chains (m, d, g1, g2, g3, g4 , a1, a2, e). A special mechanism connects one DNA segment from each category, forming a VDJC sequence, which serves as a functional gene on which the mRNA encoding the entire heavy chain is formed. The light chains are composed similarly, except that they do not contain D segments. The T receptor beta chain gene also contains multiple V, D, J, and C genes encoding the heavy chain, while the T receptor alpha chain gene The receptor contains only multiple V and J segments with a single C segment.
RESULTS OF INTERACTION OF ANTIBODIES WITH ANTIGENS
Antibodies can participate in the following reactions:
Precipitation;
Agglutinations;
Opsonization;
Neutralization;
Cellular cytotoxicity;
Cell destruction with the participation of complement.
Most immunoglobulins (antibodies) have a direct effect on the antigens with which they specifically react; for example, the formation of large aggregates may lead to precipitation or agglutination. When the antigen is a toxin, the antigen-antibody interaction can cause neutralization of the toxic effect.
In some cases, the accumulation of antibodies on the surface of the antigenic particle (opsonization) causes an increase in the phagocytic activity of macrophages and neutrophils, which have Fc receptors on their surface. This process is called immune phagocytosis.
The interaction between antigen and antibody can cause structural damage in the Fc fragment of the immunoglobulin molecule, which leads to complement activation.
COMPLEMENT
Activation of complement. Complement is a system of plasma proteins (C1-C9) that exist in an inactive form and make up approximately 10% of blood globulins. Activation of complement can occur in one of 2 ways (Fig. 10.5):
A. Classic way: the classical pathway of complement activation begins when IgM or IgG interacts with an antigen. The interaction of an antibody with an antigen leads to the fixation of C1 to the Fc part of the antibody molecule. In this case, C1q is formed and a cascade reaction occurs (Fig. 10.5). The early components (C1, 4, 2) form the C3 convertase, which cleaves C3. The final C56789 complex exhibits phospholipase activity and results in cell membrane lysis (note that the complete sequence is 1, 4, 2, 3, 5, 6, 7, 8, 9).
B. Alternative pathway (properdine pathway): the alternative pathway differs from the classical pathway only in the mechanism of activation and early reactions. C3 cleavage alternative path does not require interaction of antigen with antibodies or the presence of early (C1, C4, C2) complement factors. The cascade is triggered by aggregated IgG complexes, complex carbohydrates and bacterial endotoxins. C3 convertase is formed by the interaction of properdin (serum globulin), two other serum factors (B and D) and magnesium ions. The activation sequence after C3 cleavage is the same as in the classical pathway.
Complement activation results: Complement activation is associated with an acute inflammatory response characterized by vasodilation, increased vascular permeability, and fluid exudation, mediated by the anaphylotoxic effects of C3a and C5a. Both C3a and C5a have a strong chemotactic effect on neutrophils, which emigrate to the area of inflammation. The antigen is removed by 1) immune phagocytosis, which is caused by the opsonizing influence of C3b, neutrophils and macrophages, or 2) membrane lysis, which causes the end product of the complement cascade.
Complement receptors: complement receptors were found on the surface of most cells. CD11 is a neutrophil and macrophage receptor for C3b. CD21 is a B cell receptor for C3b. CD35 is the most widespread receptor for C3b, found on red and white blood cells; it binds immune complexes in plasma.
TYPES OF IMMUNE RESPONSE
Based on whether the immune system has been previously exposed to the antigen or not, two types of immune response are distinguished: primary and secondary.
Primary immune response
The primary immune response occurs upon first encounter with a specific antigen. Although the antigen is recognized almost immediately after entering the body, several days pass before enough immunoglobulin is produced for an increase in serum immunoglobulin levels to be detected. During this latent period, those B cells whose receptors have reacted with a specific antigen undergo six to eight successive division cycles before a sufficiently large clone of plasma cells is formed that secretes antibodies. IgM is the first immunoglobulin produced during the primary response; IgG is then produced. The switch from the synthesis of IgM to IgG or other immunoglobulins occurs as a normal event during B cell activation and occurs as a result of a switch in heavy chain genes.
Immunological memory
Memory is an essential component of the immune response because it provides an enhanced, more effective response to the second and subsequent exposures of the antigen to the body.
The mechanism underlying immunological memory has not been fully established. After antigen stimulation, lymphocyte proliferation occurs (clone expansion), which leads to the formation of a large number of executive cells (plasma cells in the B-cell system; cytotoxic T cells in the T-cell system), as well as other small lymphocytes that re-enter mitotic cycle and serve to replenish the group of cells carrying the corresponding receptor. It is assumed that since these cells are the result of antigen-induced proliferation, they are capable of an enhanced response when they encounter the antigen again (that is, they act as memory cells). In the B cell family, these cells may also undergo a switch in synthesis from IgM to IgG, which explains the immediate production of IgG by these cells during the secondary immune response.
Secondary immune response
A secondary immune response occurs when the antigen is encountered again. Re-recognition occurs immediately and the production of serum immunoglobulins, detected by laboratory tests, occurs more quickly (within 2-3 days) than with the initial response. IgG is the main immunoglobulin secreted during the secondary response. In addition, the peak level is higher and the decline occurs more slowly than in the primary response.
The ability to produce a specific secondary response is a function of immunological memory. This specific response must be differentiated from the nonspecific increase in immunoglobulin levels (against antigens other than the original antigen) that may occur after antigen stimulation - this is the so-called anamnestic response, which probably represents the random stimulation of some B cells by lymphokines arising from a specific answer.
Specificity - this is the ability of an antigen to interact with strictly defined antibodies or antigen receptors of lymphocytes.
In this case, the interaction does not occur with the entire surface of the antigen, but only with its small section, which is called the “antigenic determinant” or “epitope”. One antigen molecule can have from several units to several hundred epitopes of varying specificity. The number of epitopes determines the valency of the antigen. For example: egg albumin (M 42,000) has 5 epitopes, i.e. 5-valentene, thyroglobulin protein (M 680,000) - 40-valentene.
In protein molecules, the epitope (antigenic determinant) is formed by a set of amino acid residues. The size of the antigenic determinant of proteins can include from 5 - 7 to 20 amino acid residues. Epitopes that are recognized by antigen receptors of B and T lymphocytes have their own characteristics.
B-cell epitopes of conformational type (formed by amino acid residues from various parts protein molecule, but close in the spatial configuration of the protein globule) are located on the outer surface of the antigen, forming loops and protrusions. Typically, the number of amino acids or sugars in an epitope is from 6 to 8. Antigen recognition receptors of B cells recognize the native conformation of the epitope, rather than a linear sequence of amino acid residues.
T-cell epitopes are a linear sequence of amino acid residues that make up part of an antigen and include a larger number of amino acid residues compared to B-cell epitopes. Their recognition does not require saving the spatial configuration.
Immunogenicity - the ability of an antigen to induce immune defense of the macroorganism. The degree of immunogenicity is determined by the following factors:- Foreignness . In order for a substance to act as an immunogen, it must be recognized as “not its own.” The more foreign the antigen is, that is, the less similar it is to the body’s own structures, the stronger the immune response it causes. For example, the synthesis of antibodies to bovine serum albumin is easier to induce in a rabbit than in a goat. Rabbits belong to the order of lagomorphs and are further away in phylogenetic development from the goat and bull, which belong to the artiodactyls.
- Nature of the antigen . The most powerful immunogens are proteins. Pure polysaccharides, nucleic acids and lipids have weak immunogenic properties. At the same time, lipopolysaccharides, glycoproteins, and lipoproteins are capable of sufficiently activating the immune system.
- Molecular mass . All other things being equal, the larger molecular weight of the antigen provides greater immunogenicity. Antigens are considered good immunogens if their molecular weight is more than 10 kDa. The higher the molecular weight, the more binding sites (epitopes), which leads to an increase in the intensity of the immune response.
- Solubility. Corpuscular antigens associated with cells (erythrocytes, bacteria) are usually more immunogenic. Soluble antigens (serum albumin) may also be highly immunogenic, but are cleared more quickly. To increase the time they remain in the body, necessary for the development of an effective immune response, adjuvants (depositing substances) are used. Adjuvants are substances that are used to enhance the immune response, for example, liquid paraffin, lanolin, aluminum hydroxide and phosphate, potassium alum, calcium chloride, etc.
- Chemical structure of antigen . Increasing the number of aromatic amino acids in synthetic polypeptides increases their immunogenicity. With equal molecular weight (about 70,000), albumin is a stronger antigen than hemoglobin. At the same time, the collagen protein, whose molecular weight is 5 times greater than that of albumin and amounts to 330,000, has significantly less immunogenicity compared to albumin, which is undoubtedly due to the structural features of these proteins.
What are antigens
These are any substances contained in (or secreted by) microorganisms and other cells that carry signs of genetically foreign information and that can potentially be recognized by the body's immune system. When introduced into the internal environment of the body, these genetically foreign substances are capable of causing an immune response of various types.
Each microorganism, no matter how primitive it is, contains several antigens. The more complex its structure, the more antigens can be found in its composition.
Various elements of the microorganism have antigenic properties - flagella, capsule, cell wall, cytoplasmic membrane, ribosomes and other components of the cytoplasm, as well as various protein products released by bacteria into the external environment, including toxins and enzymes.
There are exogenous antigens (entering the body from the outside) and endogenous antigens (autoantigens - products of the body's own cells), as well as antigens that cause allergic reactions - allergens.
What are antibodies
The body continually encounters a variety of antigens. It is attacked both from the outside - from viruses and bacteria, and from the inside - from body cells that acquire antigenic properties. - serum proteins that are produced by plasma cells in response to the penetration of an antigen into the body. Antibodies are produced by cells of lymphoid organs and circulate in blood plasma, lymph and other body fluids.
The main important role of antibodies is to recognize and bind foreign material (antigen), as well as trigger the mechanism for destroying this foreign material. Essential and unique property Antibodies serve as their ability to bind antigen directly in the form in which it enters the body.
Antibodies have the ability to distinguish one antigen from another. They are capable of specific interaction with an antigen, but they interact only with the antigen (with rare exceptions) that induced their formation and fits them in spatial structure. This antibody ability is called complementarity.
A complete understanding of the molecular mechanism of antibody formation does not yet exist. The molecular and genetic mechanisms underlying the recognition of millions of different antigens found in the environment have not been studied.
Antibodies and immunoglobulins
At the end of the 30s of the 20th century, the study of the molecular nature of antibodies began. One of the methods for studying molecules was electrophoresis, which was introduced into practice in the same years. Electrophoresis allows you to separate proteins according to their electric charge and molecular weight. Serum protein electrophoresis usually produces 5 main bands, which correspond (from + to -) to the albumin, alpha1, alpha2, beta and gamma globulin fractions.
In 1939, Swedish chemist Arne Tiselius and American immunochemist Alvin Kabat used electrophoresis to fractionate the blood serum of immunized animals. Scientists have shown that antibodies are contained in a certain fraction of serum proteins. Namely, antibodies relate mainly to gamma globulins. Since some also fell into the area of beta globulins, a better term was proposed for antibodies - immunoglobulins.
In accordance with international classification, a collection of serum proteins that have the properties of antibodies is called immunoglobulins and are designated by the symbol Ig (from the word “Immunoglobulin”).
Term "immunoglobulins" reflects chemical structure molecules of these proteins. Term "antibody" determines the functional properties of the molecule and takes into account the ability of the antibody to react only with a specific antigen.
Previously, it was assumed that immunoglobulins and antibodies were synonyms. Currently, there is an opinion that all antibodies are immunoglobulins, but not all immunoglobulin molecules have the function of antibodies.
We talk about antibodies only in relation to the antigen, i.e. if the antigen is known. If we do not know the antigen complementary to a certain immunoglobulin that we have in our hands, then we only have an immunoglobulin. In any antiserum, in addition to antibodies against a given antigen, there is a large number of immunoglobulins, the antibody activity of which could not be detected, but this does not mean that these immunoglobulins are not antibodies to any other antigens. The question of the existence of immunoglobulin molecules that initially do not have the properties of antibodies remains open.
Antibodies (AT, immunoglobulins, IG, Ig) are the central figure of humoral immunity. The main role in the body's immune defense is played by lymphocytes, which are divided into two main categories - T-lymphocytes and B-lymphocytes.
Antibodies or immunoglobulins (Ig) are synthesized by B lymphocytes, or more precisely by antibody-forming cells (AFC). Antibody synthesis begins in response to antigens entering the internal environment of the body. To synthesize antibodies, B cells require contact with an antigen and the resulting maturation of B cells into antibody-forming cells. A significant number of antibodies are produced by so-called plasma cells formed from B-lymphocytes - AOC, which are detected in the blood and tissues. Immunoglobulins are contained in large quantities in serum, intercellular fluid and other secretions, providing a humoral response.
Immunoglobulin classes
Immunoglobulins (Ig) differ in structure and function. There are 5 different classes of immunoglobulins found in humans: IgG,IgA,IgM,IgE,IgD, some of which are further divided into subclasses. There are subclasses for immunoglobulins of classes G (Gl, G2, G3, G4), A (A1, A2) and M (M1, M2).
Classes and subclasses taken together are called isotypes immunoglobulins.
Antibodies of different classes differ in molecular size, charge of the protein molecule, amino acid composition and content of the carbohydrate component. The most studied class of antibodies is IgG.
In human blood serum, immunoglobulins of the IgG class normally predominate. They constitute approximately 70–80% of the total serum antibodies. IgA content - 10-15%, IgM - 5-10%. The content of immunoglobulins of the IgE and IgD classes is very small - about 0.1% for each of these classes.
One should not think that antibodies against a particular antigen belong only to one of the five classes of immunoglobulins. On the contrary, antibodies against the same antigen can be represented by different classes of Ig.
The most important diagnostic role is played by the determination of antibodies of classes M and G, since after a person is infected, class M antibodies appear first, then class G, and immunoglobulins A and E appear last.
Immunogenicity and antigenicity of antigens
In response to the entry of antigens into the body, a whole complex of reactions begins, aimed at freeing the internal environment of the body from the products of foreign genetic information. This set of protective reactions of the immune system is called immune response.
Immunogenicity is called the ability of an antigen to cause an immune response, that is, to induce a specific protective reaction of the immune system. Immunogenicity can also be described as the ability to create immunity.
Immunogenicity largely depends on the nature of the antigen, its properties (molecular weight, mobility of antigen molecules, shape, structure, ability to change), on the route and mode of entry of the antigen into the body, as well as additional influences and the genotype of the recipient.
As mentioned above, one of the forms of response of the immune system in response to the introduction of an antigen into the body is the biosynthesis of antibodies. Antibodies are able to bind the antigen that caused their formation, and thereby protect the body from the possible harmful effects of foreign antigens. In this regard, the concept of antigenicity is introduced.
Antigenicity- this is the ability of an antigen to specifically interact with immune factors, namely, to interact with the products of the immune response caused by this particular substance (antibodies and T- and B-antigen-recognition receptors).
Some terms of molecular biology
Lipids(from ancient Greek λίπος - fat) - a large group of fairly diverse natural organic compounds, including fats and fat-like substances. Lipids are found in all living cells and are one of the main components of biological membranes. They are insoluble in water and highly soluble in organic solvents. Phospholipids- complex lipids containing higher fatty acids and a phosphoric acid residue.
Conformation molecules (from Latin conformatio - shape, structure, arrangement) - geometric forms that molecules of organic compounds can take when rotating atoms or groups of atoms (substituents) around simple bonds while maintaining the order of the chemical bond of atoms unchanged ( chemical structure), bond lengths and bond angles.
Organic compounds(acids) of a special structure. Their molecules simultaneously contain amino groups (NH 2) and carboxyl groups (COOH). All amino acids consist of only 5 chemical elements: C, H, O, N, S.
Peptides(Greek πεπτος - nutritious) - a family of substances whose molecules are built from two or more amino acid residues connected into a chain by peptide (amide) bonds. Peptides whose sequence is longer than about 10-20 amino acid residues are called polypeptides.
In the polypeptide chain there are N-terminus, formed by a free α-amino group and C-end, having a free α-carboxyl group. Peptides are written and read from N-terminal to C-terminal - from N-terminal amino acid to C-terminal amino acid.
Amino acid residues- These are monomers of amino acids that make up peptides. An amino acid residue that has a free amino group is called N-terminal and is written on the left, and one that has a free α-carboxyl group is called C-terminal and is written on the right.
Proteins usually called polypeptides containing approximately 50 amino acid residues. The term “proteins” is also used as a synonym for the term “proteins” (from the Greek protos - first, most important). The molecule of any protein has a clearly defined, fairly complex, three-dimensional structure.
Amino acid residues in proteins are usually designated using a three-letter or one-letter code. The three-letter code is an abbreviation for English names amino acids and is often used in scientific literature. Single-letter codes, for the most part, do not have an intuitive connection to amino acid names and are used in bioinformatics to represent amino acid sequences in text for easy computer analysis.
Peptide backbone. In the polypeptide chain, the sequence of atoms -NH-CH-CO- is repeated many times. This sequence forms the peptide backbone. The polypeptide chain consists of a polypeptide backbone (skeleton), which has a regular, repeating structure, and individual side groups (R-groups).
Peptide bonds combine amino acids into peptides. Peptide bonds are formed by the interaction of the α-carboxyl group of one amino acid and the α-amino group of a subsequent amino acid. Peptide bonds are very strong and do not spontaneously break under normal conditions existing in cells.
Groups of atoms -CO-NH- that are repeated many times in peptide molecules are called peptide groups. The peptide group has a rigid planar (flat) structure.
Protein conformation- location of the polypeptide chain in space. The spatial structure characteristic of a protein molecule is formed due to intramolecular interactions. Due to the interaction of functional groups of amino acids, linear polypeptide chains of individual proteins acquire a certain three-dimensional structure, which is called “protein conformation.”
The process of formation of a functionally active protein conformation is called folding. Rigidity peptide bond reduces the number of degrees of freedom of the polypeptide chain, which plays an important role in the folding process.
Globular and fibrillar proteins. The proteins studied to date can be divided into two large classes according to their ability to accept a certain amount of protein in solution. geometric shape: fibrillar(stretched into a thread) and globular(rolled into a ball). The polypeptide chains of fibrillar proteins are elongated, located parallel to each other and form long threads or layers. In globular proteins, polypeptide chains are tightly folded into globules - compact spherical structures.
It should be noted that the division of proteins into fibrillar and globular is conventional, since there are a large number of proteins with an intermediate structure.
Primary protein structure(primary structure of protein) is a linear sequence of amino acids that make up a protein in a polypeptide chain. Amino acids are connected to each other by peptide bonds. The amino acid sequence is written starting from the C-terminus of the molecule, towards the N-terminus of the polypeptide chain.
P.s.b - this is the simplest level structural organization protein molecule. First P.s.b. was established by F. Sanger for insulin ( Nobel Prize for 1958).
(secondary structure of protein) - the folding of the polypeptide chain of a protein as a result of the interaction between closely spaced amino acids within the same peptide chain - between amino acids located a few residues apart from each other.
Secondary structure proteins is a spatial structure that is formed as a result of interactions between the functional groups that make up the peptide backbone.
The secondary structure of proteins is determined by the ability of peptide bond groups to undergo hydrogen interactions between the -C=O and -NH- functional groups of the peptide backbone. In this case, the peptide tends to adopt a conformation with the formation of the maximum number of hydrogen bonds. However, the possibility of their formation is limited by the nature of the peptide bond. Therefore, the peptide chain does not acquire an arbitrary, but a strictly defined conformation.
The secondary structure is formed from segments of the polypeptide chain that participate in the formation of a regular network of hydrogen bonds.
In other words, the secondary structure of a polypeptide refers to the conformation of its main chain (backbone) without taking into account the conformation of side groups.
The polypeptide chain of a protein, folding under the influence of hydrogen bonds into a compact form, can form a number of regular structures. Several such structures are known: α (alpha)-helix, β (beta)-structure (another name is β-pleated layer or β-pleated sheet), random coil and turn. A rare type of protein secondary structure is π-helices. Initially, researchers believed that this type of helix did not occur in nature, but later these helices were discovered in proteins.
The α-helix and β-structure are the energetically most favorable conformations, since they are both stabilized by hydrogen bonds. In addition, both the α-helix and β-structure are further stabilized by the close packing of the backbone atoms, which fit together like pieces of a picture puzzle.
These fragments and their combination in a certain protein, if present, are also called the secondary structure of this protein.
In the structure of globular proteins, fragments of a regular structure of all types can be found in any combination, but there may not be any. In fibrillar proteins, all residues belong to one type: for example, wool contains α-helices, and silk contains β-structures.
Thus, most often the secondary structure of a protein is the folding of the protein polypeptide chain into α-helical regions and β-structural formations (layers) involving hydrogen bonds. If hydrogen bonds are formed between the bending areas of one chain, then they are called intrachain; if between chains, they are called interchain. Hydrogen bonds are located perpendicular to the polypeptide chain.
α-helix-formed by intrachain hydrogen bonds between the NH group of one amino acid residue and the CO group of the fourth residue from it. The average length of α-helices in proteins is 10 amino acid residues
In an α-helix, hydrogen bonds are formed between the oxygen atom of the carbonyl group and the hydrogen of the amide nitrogen of the 4th amino acid from it. All C=O and N-H groups of the main polypeptide chain are involved in the formation of these hydrogen bonds. The side chains of amino acid residues are located along the periphery of the helix and do not participate in the formation of the secondary structure.
β-structures are formed between the linear regions of the peptide backbone of one polypeptide chain, thereby forming folded structures (several zigzag polypeptide chains).
The β-structure is formed due to the formation of many hydrogen bonds between the atoms of the peptide groups of linear chains. In β-structures, hydrogen bonds are formed between primary structure amino acids or different protein chains, and not closely spaced, as is the case in an α-helix.
In some proteins, β-structures can be formed due to the formation of hydrogen bonds between atoms of the peptide backbone of different polypeptide chains.
Polypeptide chains or parts thereof can form parallel or antiparallel β-structures. If several chains of a polypeptide are connected in opposite directions, and the N- and C-termini do not coincide, then antiparallelβ-structure, if they coincide – parallelβ-structure.
Another name for β-structures is β-sheets(β-folded layers, β-sheets). A β-sheet is formed from two or more β-structural regions of a polypeptide chain called β-strands. Typically, β-sheets are found in globular proteins and contain no more than 6 β-strands.
β-strands(β-strands) are regions of a protein molecule in which the bonds of the peptide backbone of several consecutive polypeptides are organized in a planar conformation. In illustrations, the β-strands of proteins are sometimes depicted as flat "arrowhead bands" to emphasize the direction of the polypeptide chain.
The main part of the β-strands is located adjacent to other strands and forms with them an extensive system of hydrogen bonds between the C=O and N-H groups of the main protein chain (peptide backbone). β-strands can be packaged
A messy tangle- this is a section of the peptide chain that does not have any regular, periodic spatial organization. Such regions in each protein have their own fixed conformation, which is determined by the amino acid composition of this region, as well as the secondary and tertiary structures of adjacent regions surrounding the “chaotic coil”. In regions of a random coil, the peptide chain can bend relatively easily and change conformation, while the α-helices and β-sheet layer are fairly rigid structures
Another form of secondary structure is denoted as β-turn. This structure is formed by 4 or more amino acid residues with a hydrogen bond between the first and last, and in such a way that the peptide chain changes direction by 180°. The loop structure of such a turn is stabilized by a hydrogen bond between the carbonyl oxygen of the amino acid residue at the beginning of the turn and N-H group the third residue along the chain at the end of the turn.
If antiparallel β-strands approach the β-turn from both ends, then a secondary structure is formed, called β-hairpin(β-hairpin)
Protein tertiary structure(tertiary structure of protein) - In solution under physiological conditions, the polypeptide chain folds into a compact formation that has a certain spatial structure, which is called the tertiary structure of the protein. It is formed as a result of self-folding due to interactions between radicals (covalent and hydrogen bonds, ionic and hydrophobic interactions). For the first time T.s.b. was established for the myoglobin protein by J. Kendrew and M. Perutz in 1959 (Nobel Prize for 1962). T.s.b. almost completely determined by the primary structure of the protein. Currently, using the methods of X-ray diffraction analysis and nuclear magnetic spectroscopy (NMR spectroscopy), the spatial (tertiary) structures of a large number of proteins have been determined.
Quaternary structure of protein. Proteins consisting of one polypeptide chain have only tertiary structure. However, some proteins are built from several polypeptide chains, each of which has a tertiary structure. For such proteins, the concept of quaternary structure has been introduced, which is the organization of several polypeptide chains with a tertiary structure into a single functional protein molecule. Such a protein with a quaternary structure is called an oligomer, and its polypeptide chains with a tertiary structure are called protomers or subunits.
Conjugate(conjugate, lat. conjugatio - connection) - an artificially synthesized (chemically or by recombination in vitro) hybrid molecule in which two molecules with different properties are connected (combined); widely used in medicine and experimental biology.
Haptens
Haptens- these are “defective antigens” (the term was proposed by the immunologist K. Landsteiner). When introduced into the body under normal conditions, haptens are not capable of inducing an immune response in the body, since they have extremely low immunogenicity.
Most often, haptens are low molecular weight compounds (molecular weight less than 10 kDa). They are recognized by the recipient's body as genetically foreign (i.e., they have specificity), but due to their low molecular weight, they do not themselves cause immune reactions. However, they have not lost their antigenic property, which allows them to specifically interact with ready-made immune factors (antibodies, lymphocytes).
Under certain conditions, it is possible to force the immune system of the macroorganism to specifically respond to the hapten as a full-fledged antigen. To do this, it is necessary to artificially enlarge the hapten molecule - to connect it with a strong bond to a sufficiently large protein molecule or other carrier polymer. The conjugate synthesized in this way will have all the properties of a full-fledged antigen and cause an immune response when introduced into the body.
Epitopes (antigenic determinants)
The body can form antibodies to almost any part of the antigen molecule, but this usually does not happen during a normal immune response. Complex antigens (proteins, polysaccharides) have special areas to which a specific immune response is actually formed. Such areas are called epitopes(epitope), from Greek. epi - on, above, over and topos - place, area. Synonym - antigenic determinant.
These sections consist of a few amino acids or carbohydrates, each section is a group of amino acid residues of a protein antigen or a section of a polysaccharide chain. Epitopes are able to interact both with specific lymphocyte receptors, thereby inducing an immune response, and with antigen-binding centers of specific antibodies.
Epitopes are diverse in their structure. An antigenic determinant (epitope) can be a region of the protein surface formed by amino acid radicals, a hapten or a prosthetic group of a protein (a non-protein component associated with a protein), especially often polysaccharide groups of glycoproteins.
Antigenic determinants or epitopes are specific regions of the three-dimensional structure of antigens. There are different types of epitopes - linear And conformational.
Linear epitopes are formed by a linear sequence of amino acid residues.
As a result of studying the structure of proteins, it was found that protein molecules have a complex spatial structure. When coiled (into a ball), protein macromolecules can bring together residues that are distant from each other in a linear sequence, forming a conformational antigenic determinant.
In addition, there are terminal epitopes (located at the ends of the antigen molecule) and central ones. “Deep,” or hidden, antigenic determinants, which appear when the antigen is destroyed, are also determined.
The molecules of most antigens are quite large. One protein macromolecule (antigen), consisting of several hundred amino acids, can contain many different epitopes. Some proteins may have the same antigenic determinant in multiple copies (repeated antigenic determinants).
A wide range of different antibodies are formed against one epitope. Each of the epitopes is capable of stimulating the production of different specific antibodies. Specific antibodies can be produced for each of the epitopes.
There is a phenomenon immunodominance, which manifests itself in the fact that epitopes differ in their ability to induce an immune response.
Not all epitopes in a protein are characterized by equal antigenicity. As a rule, some epitopes of an antigen have special antigenicity, which is manifested in the preferential formation of antibodies against these epitopes. A hierarchy is established in the spectrum of epitopes of the protein molecule - some of the epitopes are dominant and most antibodies are formed specifically to them. These epitopes are named immunodominant epitopes. They are almost always located on prominent parts of the antigen molecule.
Structure of antibodies (immunoglobulins)
IgG immunoglobulins based on experimental data. Each amino acid residue of a protein molecule is depicted as a small ball. Visualization was built using the RasMol program.
During the 20th century, biochemists sought to find out what variants of immunoglobulins exist and what is the structure of the molecules of these proteins. The structure of antibodies was established through various experiments. Basically, they consisted in the fact that the antibodies were treated with proteolytic enzymes (papain, pepsin), and were subjected to alkylation and reduction with mercaptoethanol.
Then the properties of the resulting fragments were studied: their molecular weight (by chromatography), quaternary structure (by X-ray diffraction analysis), ability to bind to antigen, etc. was determined. Antibodies to these fragments were also used to determine whether antibodies to one type of fragment could bind to fragments of another type. Based on the data obtained, a model of the antibody molecule was built.
More than 100 years of research into the structure and function of immunoglobulins has only emphasized the complex nature of these proteins. Currently, the structure of human immunoglobulin molecules has not been fully described. Most researchers have concentrated their efforts not on describing the structure of these proteins, but on elucidating the mechanisms by which antibodies interact with antigens. In addition, antibody molecules
Despite the supposed diversity of immunoglobulins, their molecules have been classified according to the structures included in these molecules. This classification is based on the fact that immunoglobulins of all classes are built according to a general plan and have a certain universal structure.
Immunoglobulin molecules are complex spatial formations. All antibodies, without exception, belong to the same type of protein molecules that have a globular secondary structure, which corresponds to their name - “immunoglobulins” (the secondary structure of a protein is the way its polypeptide chain is laid out in space). They can be monomers or polymers built from several subunits.
Heavy and light polypeptide chains in the structure of immunoglobulins
Peptide chains of immunoglobulins. Schematic illustration. Variable regions are highlighted with dotted lines.
The structural unit of immunoglobulin is a monomer, a molecule consisting of polypeptide chains connected to each other by disulfide bonds (S-S bridges).
If an Ig molecule is treated with 2-mercaptoethanol (a reagent that destroys disulfide bonds), it will disintegrate into pairs of polypeptide chains. The resulting polypeptide chains are classified by molecular weight: light and heavy. Light chains have a low molecular weight (about 23 kDa) and are designated by the letter L, from the English. Light - light. Heavy chains H (from the English Heavy - heavy) have a high molecular weight (varies between 50 - 73 kDa).
The so-called monomeric immunoglobulin contains two L chains and two H chains. The light and heavy chains are held together by disulfide bridges. Disulfide bonds connect light chains to heavy chains and heavy chains to each other.
The main structural subunit of all classes of immunoglobulins is the light chain-heavy chain (L-H) pair. The structure of immunoglobulins of different classes and subclasses differs in the number and location of disulfide bonds between heavy chains, as well as in the number of (L-H) subunits in the molecule. The H-chains are held together by varying numbers of disulfide bonds. The types of heavy and light chains that make up different classes of immunoglobulins also differ.
The figure shows a diagram of the organization of IgG as a typical immunoglobulin. Like all immunoglobulins, IgG contains two identical heavy (H) chains and two identical light (L) chains, which are linked into a four-chain molecule through interchain disulfide bonds (-S-S-). The only disulfide bond connecting the H and L chains is located near the C-terminus of the light chain. There is also a disulfide bond between the two heavy chains.
Domains within an antibody molecule
The light and heavy polypeptide chains in the Ig molecule have a specific structure. Each chain is conventionally divided into specific sections called domains.
Both light and heavy chains do not form a straight thread. Within each chain, at regular and approximately equal intervals of 100-110 amino acids, there are disulfide bridges that form loops in the structure of each chain. The presence of disulfide bridges means that each loop in the peptide chains must form a compactly folded globular domain. Thus, each polypeptide chain in the immunoglobulin forms several globular domains in the form of loops, including approximately 110 amino acid residues.
We can say that immunoglobulin molecules are assembled from separate domains, each of which is located around a disulfide bridge and is homologous to the others.
In each of the light chains of antibody molecules, there are two intrachain disulfide bonds; accordingly, each light chain has two domains. The number of such bonds in heavy chains varies; heavy chains contain four or five domains. Domains are separated by easily organized segments. The presence of such configurations was confirmed by direct observations and genetic analysis.
Primary, secondary, tertiary and quaternary structure of immunoglobulins
The structure of the immunoglobulin molecule (as well as other proteins) is determined by the primary, secondary, tertiary and quaternary structure. The primary structure is the sequence of amino acids that make up the light and heavy chains of immunoglobulins. X-ray diffraction analysis showed that the light and heavy chains of immunoglobulins consist of compact globular domains (the so-called immunoglobulin domains). The domains are arranged in a characteristic tertiary structure called the immunoglobulin fold.
Immunoglobulin domains are regions in the tertiary structure of the Ig molecule that are characterized by a certain autonomy of structural organization. Domains are formed by different segments of the same polypeptide chain, folded into “balls” (globules). The globule contains approximately 110 amino acid residues.
Domains have similar general structure and specific functions to each other. Within the domains, the peptide fragments that make up the domain form a compactly folded antiparallel β-sheet structure stabilized by hydrogen bonds (protein secondary structure). There are practically no regions with an α-helical conformation in the structure of the domains.
The secondary structure of each domain is formed by folding an extended polypeptide chain back and forth upon itself into two antiparallel β-sheets (β-sheets) containing several β-sheets. Each β-sheet has a flat shape - the polypeptide chains in the β-sheets are almost completely elongated.
The two β-sheets that make up the immunoglobulin domain are arranged in a structure called a β-sandwich (“like two pieces of bread on top of each other”). The structure of each immunoglobulin domain is stabilized by an intradomain disulfide bond—the β-sheets are covalently linked by a disulfide bond between the cysteine residues of each β-sheet. Each β-sheet consists of antiparallel β-strands connected by loops of varying lengths.
The domains, in turn, are interconnected by a continuation of the polypeptide chain, which extends beyond the β-sheets. The open sections of the polypeptide chain present between the globules are especially sensitive to proteolytic enzymes.
The globular domains of a light and heavy chain pair interact with each other to form a quaternary structure. Due to this, functional fragments are formed that allow the antibody molecule to specifically bind the antigen and, at the same time, perform a number of biological effector functions.
Variable and constant domains
Domains in peptide chains differ in the consistency of their amino acid composition. There are variable and constant domains (regions). Variable domains are designated by the letter V, from the English. variable - “changeable” and are called V-domains. Permanent (constant) domains are designated by the letter C, from the English constant - “permanent” and are called C-domains.
Immunoglobulins produced by different clones of plasma cells have variable domains of different amino acid sequences. The constant domains are similar or very similar for each immunoglobulin isotype.
Each domain is designated by a letter indicating whether it belongs to the light or heavy chain and a number indicating its position.
The first domain on the light and heavy chains of all antibodies is extremely variable in amino acid sequence; it is denoted as V L and V H respectively.
The second and subsequent domains on both heavy chains are much more constant in amino acid sequence. They are designated CH or C H 1, C H 2 and C H 3. Immunoglobulins IgM and IgE have an additional C H 4 domain on the heavy chain, located behind the C H 3 domain.
The half of the light chain including the carboxyl terminus is called the constant region C L , and the N-terminal half of the light chain is called the variable region V L .
Carbohydrate chains are also associated with the CH2 domain. Immunoglobulins of different classes differ greatly in the number and location of carbohydrate groups. The carbohydrate components of immunoglobulins have a similar structure. They consist of a constant core and a variable outer part. Carbohydrate components affect the biological properties of antibodies.
Fab and Fc fragments of the immunoglobulin molecule
The variable domains of the light and heavy chains (V H and V L), together with the constant domains closest to them (C H 1 and C L 1), form Fab fragments of antibodies (fragment, antigen binding). The immunoglobulin region that binds to a specific antigen is formed by the N-terminal variable regions of the light and heavy chains, i.e. V H - and V L -domains.
The remaining part, represented by the C-terminal constant domains of the heavy chains, is designated as the Fc fragment (fragment, crystallizable). The Fc fragment includes the remaining CH domains held together by disulfide bonds. At the junction of the Fab and Fc fragments there is a hinge region that allows the antigen-binding fragments to unfold for closer contact with the antigen.
Hinge area
At the border of the Fab and Fc fragments there is the so-called. "hinge area" having a flexible structure. It provides mobility between the two Fab fragments of the Y-shaped antibody molecule. The mobility of antibody molecule fragments relative to each other is an important structural characteristic of immunoglobulins. This type of interpeptide connection makes the structure of the molecule dynamic - it allows you to easily change the conformation depending on the surrounding conditions and state.
The hinge region is a section of the heavy chain. The hinge region contains disulfide bonds that connect the heavy chains to each other. For each class of immunoglobulins, the hinge region has its own structure.
In immunoglobulins (with the possible exception of IgM and IgE), the hinge region consists of a short segment of amino acids and is found between the C H 1 and C H 2 regions of the heavy chains. This segment consists predominantly of cysteine and proline residues. Cysteines are involved in the formation of disulfide bridges between chains, and proline residues prevent folding into a globular structure.
Typical structure of an immunoglobulin molecule using IgG as an example
The schematic representation in the planar drawing does not accurately reflect the structure of Ig; in reality, the variable domains of the light and heavy chains are not arranged in parallel, but are closely intertwined with each other in a criss-cross pattern.
It is convenient to consider the typical structure of an immunoglobulin using the example of an IgG antibody molecule. There are a total of 12 domains in the IgG molecule - 4 on the heavy chains and 2 on the light chains.
Each light chain includes two domains - one variable (V L, variable domain of the light chain) and one constant (CL, constant domain of the light chain). Each heavy chain contains one variable domain (V H, variable domain of the heavy chain) and three constant domains (CH 1–3, constant domains of the heavy chain). About a quarter of the heavy chain, including the N-terminus, is classified as the variable region of the H chain (VH), the rest of it is the constant region (CH1, CH2, CH3).
Each pair of variable domains V H and V L located in adjacent heavy and light chains forms a variable fragment (Fv, variable fragment).
Types of heavy and light chains in antibody molecules
Based on differences in the primary structure of permanent regions, circuits are divided into types. The types are determined by the primary amino acid sequence of the chains and the degree of glycosylation. Light chains are divided into two types: κ and λ (kappa and lambda), heavy chains are divided into five types: α, γ, μ, ε and δ (alpha, gamma, mu, epsilon and delta). Among the variety of heavy chains of alpha, mu and gamma types, subtypes are distinguished.
Classification of immunoglobulins
Immunoglobulins are classified according to their H-chain (heavy chain) type. The constant regions of the heavy chains of immunoglobulins of different classes are not the same. Human immunoglobulins are divided into 5 classes and a number of subclasses, according to the types of heavy chains that are included in their composition. These classes are called IgA, IgG, IgM, IgD and IgE.
The H-chains themselves are designated by a Greek letter, corresponding to the capital Latin letter of the name of one of the immunoglobulins. IgA has heavy chains α (alpha), IgM – μ (mu), IgG – γ (gamma), IgE – ε (epsilon), IgD – δ (delta).
Immunoglobulins IgG, IgM and IgA have a number of subclasses. Division into subclasses (subtypes) also occurs depending on the characteristics of the H-chains. In humans, there are 4 subclasses of IgG: IgG1, IgG2, IgG3 and IgG4, containing heavy chains γ1, γ2, γ3 and γ4, respectively. These H chains differ in small Fc fragment details. For the μ-chain, 2 subtypes are known - μ1- and μ2-. IgA has 2 subclasses: IgA1 and IgA2 with α1 and α2 subtypes of α chains.
In each immunolobulin molecule, all heavy chains are of the same type, in accordance with the class or subclass.
All 5 classes of immunoglobulins consist of heavy and light chains.
The light chains (L-chains) of immunoglobulins of different classes are the same. All immunoglobulins can have either both κ (kappa) or both λ (lambda) light chains. Immunoglobulins of all classes are divided into K- and L-types, depending on the presence of κ- or λ-type light chains in their molecules, respectively. In humans, the ratio of K- and L-types is 3:2.
The classes and subclasses taken together are called immunoglobulin isotypes. The antibody isotype (class, subclass of immunoglobulins - IgM1, IgM2, IgG1, IgG2, IgG3, IgG4, IgA1, IgA2, IgD, IgE) is determined by the C-domains of the heavy chains.
Each class includes a huge variety of individual immunoglobulins, differing in the primary structure of the variable regions; the total number of immunoglobulins of all classes is ≈ 10^7.
The structure of antibody molecules of various classes
Schemes of the structure of immunoglobulins. (A) - monomeric IgG, IgE, IgD, IgA; (B) - polymeric secretory Ig A (slgA) and IgM (B); (1) - secretory component; (2) - connecting J-chain.
1. Antibody classes IgG, IgD and IgE
Antibody molecules of the IgG, IgD and IgE classes are monomeric; they are Y-shaped.
IgG class immunoglobulins account for 75% of the total number of human immunoglobulins. They are found both in the blood and outside the blood vessels. An important property of IgG is its ability to pass through the placenta. Thus, maternal antibodies enter the body of the newborn child and protect him from infection in the first months of life (natural passive immunity).
IgD is mainly found on the membrane of B lymphocytes. They have a structure similar to IgG, 2 active centers. The heavy chain (δ chain) consists of a variable and 3 constant domains. The hinge region of the δ chain is the longest, and the location of carbohydrates in this chain is also unusual.
IgE - the concentration of this class of immunoglobulins in blood serum is extremely low. IgE molecules are mainly fixed on the surface of mast cells and basophils. IgE is similar in structure to IgG and has 2 active centers. The heavy chain (ε-chain) has one variable and 4 constant domains. It is assumed that IgE is essential in the development of anthelmintic immunity. IgE plays a major role in the pathogenesis of some allergic diseases (bronchial asthma, hay fever) and anaphylactic shock.
2. Antibody classes IgM and IgA
Immunoglobulins IgM and IgA form polymer structures. For polymerization, IgM and IgA include an additional polypeptide chain with a molecular weight of 15 kDa, called the J-chain (joint). This J-chain binds the terminal cysteines at the C-termini of the μ- and α-heavy chains of IgM and IgA, respectively.
On the surface of mature B lymphocytes, IgM molecules are located in the form of monomers. However, in serum they exist in the form of pentamers: the IgM molecule consists of five structural molecules arranged radially. The IgM pentamer is formed from five “slingshot” monomers, similar to IgG, linked together by disulfide bonds and a J chain. Their Fc fragments are directed to the center (where they are connected by a J-chain), and their Fab fragments are directed outward.
In IgM, the heavy (H) chains consist of 5 domains, since they contain 4 constant domains. IgM heavy chains do not have a hinge region; its role is played by the C H 2 domain, which has some conformational lability.
IgM is synthesized mainly during the primary immune response and is predominantly found in the intravascular bed. The amount of Ig M in the blood serum of healthy people is about 10% of the total amount of Ig.
IgA antibodies are built from varying numbers of monomers. Class A immunoglobulins are divided into two types: serum and secretory. The majority (80%) of IgA present in blood serum has a monomeric structure. Less than 20% of IgA in serum is represented by dimeric molecules.
Secretory IgA is not found in the blood, but as part of exocretes on the mucous membranes and is designated sIgA. In the secretions of mucous membranes, IgA is presented in the form of dimers. Secretory IgA forms a dimer of two “slingshots” (Ig monomers). The C-termini of the heavy chains in the sIgA molecule are connected to each other by the J-chain and a protein molecule called the “secretory component”.
The secretory component is produced by epithelial cells of the mucous membranes. It attaches to the IgA molecule as it passes through epithelial cells. The secretory component protects sIgA from cleavage and inactivation by proteolytic enzymes, which are contained in large quantities in the secretions of the mucous membranes.
The main function of sIgA is to protect mucous membranes from infection. The role of sIgA in providing local immunity is very significant, because The total area of the mucous membranes in the adult human body is several hundred square meters and far exceeds the surface of the skin.
High concentrations of sIgA are found in human breast milk, especially in the first days of lactation. They protect the newborn's gastrointestinal tract from infection.
Children are born without IgA and receive it through their mother's milk. It has been reliably shown that children who are breastfed are significantly less likely to suffer from intestinal infections and respiratory tract diseases compared to children receiving artificial nutrition.
Antibodies of the IgA class make up 15-20% of the total content of immunoglobulins. IgA does not penetrate the placental barrier. Ig A is synthesized by plasma cells located mainly in submucosal tissues, on the mucous epithelial surface of the respiratory tract, urogenital and intestinal tract, and in almost all excretory glands. Part of Ig A enters the general circulation, but most of it is secreted locally on the mucous membranes in the form of sIgA and serves as a local protective immunological barrier for the mucous membranes. Serum IgA and sIgA are different immunoglobulins; sIgA is not found in blood serum.
People with IgA immunodeficiency have a tendency to autoimmune diseases, infections of the respiratory tract, maxillary and frontal sinuses, and intestinal disorders.
Digestion of the immunoglobulin molecule by enzymes
Proteolytic enzymes (such as papain or pepsin) break down immunoglobulin molecules into fragments. At the same time, under the influence of different proteases, different products can be obtained. Immunoglobulin fragments obtained in this way can be used for research or medical purposes.
The globular structure of immunoglobulins and the ability of enzymes to break down these molecules into large components in strictly defined places, and not destroy them into oligopeptides and amino acids, indicates an extremely compact structure.
1. Cleavage of the immunoglobulin molecule by papain. Fab and Fc fragments of antibodies.
In the late 50s - early 60s, the English scientist R.R. Porter analyzed the structural characteristics of IgG antibodies by separating the molecule with papain (a purified enzyme from papaya juice). Papain destroys immunoglobulin in the hinge region, above the interchain disulfide bonds. This enzyme splits the immunoglobulin molecule into three fragments of approximately the same size.
Two of them were named Fab fragments(from the English fragment antigen-binding - antigen-binding fragment). Fab fragments are completely identical and, as studies have shown, are designed to bind to antigen. The heavy chain region of the Fab fragment is called Fd; it consists of V H and C H 1 domains.
The third fragment may crystallize out of solution and cannot bind antigen. This fragment is named Fc fragment(from the English fragment crystallizable - fragment of crystallization). It is responsible for the biological functions of the antibody molecule after binding the antigen and the Fab part of the intact antibody molecule.
The Fc fragment has the same structure for antibodies of each class and subclass and different for antibodies belonging to different subclasses and classes.
The Fc fragment of the molecule interacts with cells of the immune system: neutrophils, macrophages and other mononuclear phagocytes that carry receptors for the Fc fragment on their surface. If antibodies bind to pathogenic microorganisms, they can interact with phagocytes with their Fc fragment. Thanks to this, the pathogen cells will be destroyed by these phagocytes. In fact, antibodies act in this case as intermediary molecules.
Subsequently, it became known that the Fc fragments of immunoglobulins within one isotype in a given organism are strictly identical, regardless of the antigen specificity of the antibody. For this invariance, they began to be called constant regions (fragment constant - Fc, the abbreviation is the same).
2. Cleavage of the immunoglobulin molecule by pepsin.
Another proteolytic enzyme, pepsin, cleaves the molecule at a different location, closer to the C-terminus of the H chains than papain does. Cleavage occurs “downstream” of the disulfide bonds holding the H chains together. As a result, under the action of pepsin, a divalent antigen-binding F(ab")2 fragment and a truncated pFc" fragment are formed. The pFc" fragment is the C-terminal portion of the Fc region.
Pepsin cuts the pFc" fragment from a large fragment with a sedimentation constant of 5S. This large fragment is called F(ab")2 because, like the parent antibody, it is bivalent with respect to antigen binding. It consists of linked Fab fragments linked by a disulfide bridge at the hinge region. These Fab fragments are monovalent and homologous to papain Fab fragments I and II, but their Fd fragment is approximately ten amino acid residues larger.
Antigen-binding centers of antibodies (paratopes)
The Fab fragment of immunoglobulin includes V domains of both chains, C L and C H 1 domains. The antigen-binding region of the Fab fragment has received several names: the active or antigen-binding center of antibodies, antideterminant or paratope.
Variable segments of light and heavy chains participate in the formation of active centers. The active site is a cleft located between the variable domains of the light and heavy chains. Both of these domains participate in the formation of the active center.
Immunoglobulin molecule. L - light chains; H - heavy chains; V - variable region; C - constant region; The N-terminal regions of the L and H chains (V region) form two antigen-binding centers within the Fab fragments.
Each Fab fragment of IgG immunoglobulins has one antigen-binding site. The active centers of antibodies of other classes, capable of interacting with the antigen, are also located in Fab fragments. Antibodies IgG, IgA and IgE each have 2 active centers, IgM - 10 centers.
Immunoglobulins can bind antigens of different chemical natures: peptides, carbohydrates, sugars, polyphosphates, steroid molecules.
An essential and unique property of antibodies is their ability to bind to intact, native molecules of antigens, directly in the form in which the antigen has penetrated into the internal environment of the body. This does not require any pre-metabolic processing of antigens
Structure of domains in immunoglobulin molecules
The secondary structure of the polypeptide chains of the immunoglobulin molecule has a domain structure. Individual sections of heavy and light chains are folded into globules (domains), which are connected by linear fragments. Each domain is approximately cylindrical in shape and is a β-sheet structure formed from antiparallel β-sheets. Within the basic structure, there is a distinct difference between the C and V domains, which can be seen using the light chain as an example.
The figure schematically shows the folding of a single polypeptide chain of the Bence-Jones protein containing V L and C L domains. The scheme is based on X-ray diffraction data - a method that allows you to establish the three-dimensional structure of proteins. The diagram shows the similarities and differences between the V and C domains.
The upper part of the figure schematically shows the spatial arrangement of the constant (C) and variable (V) domains of the light chain of a protein molecule. Each domain is a cylindrical “barrel-shaped” structure in which sections of the polypeptide chain (β-strands) running in opposite directions (i.e., antiparelle) are packed to form two β-sheets held together by a disulfide communication
Each of the domains, V- and C-, consists of two β-sheets (layers with a β-sheet structure). Each β-sheet contains several antiparallel (running in opposite directions) β-strands: in the C-domain the β-sheets contain four and three β-strands, in the V-domain both layers consist of four β-strands. In the figure, the β-strands are shown in yellow and green for the C domain and red and blue for the V domain.
In the lower part of the figure, immunoglobulin domains are discussed in more detail. This half of the picture shows a diagram of the relative arrangement of β-strands for the V- and C-domains of the light chain. It is possible to more clearly examine the way in which their polypeptide chains are stacked when forming β-sheets, which creates the final structure. To show the folding, the β-strands are designated by letters of the Latin alphabet, according to the order of their appearance in the sequence of amino acids that make up the domain. The order of occurrence in each β-sheet is a characteristic of immunoglobulin domains.
The β-sheets (sheets) in the domains are linked by a disulfide bridge (bond) approximately in the middle of each domain. These bonds are shown in the figure: between the layers there is a disulfide bond connecting folds B and F and stabilizing the structure of the domain.
The main difference between the V and C domains is that the V domain is larger and contains additional β-strands, designated Cʹ and Cʹʹ. In the figure, the β-strands Cʹ and Cʹʹ, present in the V-domains but absent in the C-domains, are highlighted with a blue rectangle. It can be seen that each polypeptide chain forms flexible loops between successive β-strands when changing direction. In the V domain, flexible loops formed between some of the β-strands form part of the active site structure of the immunoglobulin molecule.
Hypervariable regions within V domains
The level of variability within variable domains is not evenly distributed. Not the entire variable domain is variable in its amino acid composition, but only a small part of it - hypervariable areas. They account for about 20% of the amino acid sequence of V-domains.
In the structure of the whole immunoglobulin molecule, the V H and V L domains are combined. Their hypervariable regions are adjacent to each other and create a single hypervariable region in the form of a pocket. This is the region that specifically binds to the antigen. Hypervariable regions determine the complementarity of the antibody to the antigen.
Since hypervariable regions play a key role in antigen recognition and binding, they are also called complementarity determining regions (CDRs). There are three CDRs in the variable domains of the heavy and light chains (V L CDR1–3, V H CDR1–3).
Between the hypervariable regions are relatively constant sections of the amino acid sequence, which are called frame regions (FR). They account for about 80% of the amino acid sequence of V-domains. The role of such regions is to maintain a relatively uniform three-dimensional structure of V-domains, which is necessary to ensure affinity interaction of hypervariable regions with the antigen.
In the variable domain sequence of region 3, hypervariant regions alternate with 4 relatively invariant “framework” regions FR1–FR4,
H1–3 – CDR loops included in the chains.
Of particular interest is the spatial arrangement of the hypervariable regions in three separate loops of the variable domain. These hypervariable regions, although located at a great distance from each other in the primary structure of the light chain, but, when the three-dimensional structure is formed, they are located in close proximity to each other.
In the spatial structure of V-domains, hypervariable sequences are located in the zone of bends of the polypeptide chain, directed towards the corresponding sections of the V-domain of the other chain (i.e., the CDRs of the light and heavy chains are directed towards each other). As a result of the interaction of the variable domain of the H- and L-chains, the antigen-binding site (active center) of the immunoglobulin is formed. According to electron microscopy, it is a cavity 6 nm long and 1.2–1.5 nm wide.
The spatial structure of this cavity, determined by the structure of hypervariable regions, determines the ability of antibodies to recognize and bind specific molecules based on spatial correspondence (antibody specificity). Spatially separated regions of the H- and L-chains also contribute to the formation of the active center. The hypervariable regions of the V domains are not completely included in the active center - the surface of the antigen-binding region covers only about 30% of the CDR.
The hypervariable regions of the heavy and light chain determine the individual structural features of the antigen-binding center for each Ig clone and the diversity of their specificities.
The ultra-high variability of CDRs and active centers ensures that immunoglobulin molecules synthesized by B lymphocytes of the same clone are unique, not only in structure, but also in their ability to bind various antigens. Despite the fact that the structure of immunoglobulins is quite well known and it is the CDRs that are responsible for their features, it is still not clear which domain is most responsible for antigen binding.
Interaction of antibodies and antigens (interaction of epitope and paratope)
The antigen-antibody reaction is based on the interaction between the antigen epitope and the active center of the antibody, based on their spatial correspondence (complementarity). As a result of the binding of the pathogen to the active center of the antibody, the pathogen is neutralized and its penetration into the body's cells is difficult.
In the process of interaction with the antigen, not the entire immunoglobulin molecule takes part, but only a limited part of it - the antigen-binding center, or paratope, which is localized in the Fab fragment of the Ig molecule. In this case, the antibody does not interact with the entire antigen molecule at once, but only with its antigenic determinant (epitope).
The active center of antibodies is a structure that is spatially complementary (specific) to the determinant group of the antigen. The active center of antibodies has functional autonomy, i.e. capable of binding antigenic determinants in isolated form.
On the antigen side, epitopes that interact with specific antibodies are responsible for interaction with the active centers of antigen recognition molecules. The epitope directly enters into ionic, hydrogen, van der Waals and hydrophobic bonds with the active center of the antibody.
The specific interaction of antibodies with an antigen molecule is associated with a relatively small area of its surface, corresponding in size to the antigen-binding site of receptors and antibodies.
The binding of antigen to antibody occurs through weak interactions within the antigen-binding center. All these interactions appear only when the molecules are in close contact. Such a small distance between molecules can only be achieved due to the complementarity of the epitope and the active center of the antibody.
Sometimes the same antigen-binding site of an antibody molecule can bind to several different antigenic determinants (usually these antigenic determinants are very similar). Such antibodies are called cross-reactive, capable of polyspecific binding.
For example, if antigen A has common epitopes with antigen B, then some of the antibodies specific to A will also react with B. This phenomenon is called cross reactivity.
Complete and incomplete antibodies. Valence
Valence- this is the number of active centers of the antibody that are able to combine with antigenic determinants. Antibodies have a different number of active centers in the molecule, which determines their valence. In this regard, there is a distinction full And incomplete antibodies.
Full antibodies have at least two active centers. Full (divalent and pentavalent) antibodies, when interacting in vitro with the antigen in response to which they are produced, give visually visible reactions (agglutination, lysis, precipitation, complement fixation, etc.).
Incomplete or monovalent antibodies differ from regular (complete) antibodies in that they have only one active center; the second center does not work in such antibodies. This does not mean that the second active center of the molecule is absent. The second active center of such immunoglobulins is shielded by various structures or has low avidity. Such antibodies can interact with the antigen, block it, binding epitopes of the antigen and preventing the contact of full antibodies with it, but do not cause aggregation of the antigen. Therefore they are also called blocking.
The reaction between partial antibodies and antigen is not accompanied by macroscopic phenomena. Incomplete antibodies, when specifically interacting with a homologous antigen, do not give a visible manifestation of a serological reaction, because cannot aggregate particles into large conglomerates, but only block them.
Incomplete antibodies are formed independently of complete ones and perform the same functions. They are also represented by different classes of immunoglobulins.
Idiotypes and idiotopes
Antibodies are complex protein molecules that themselves can have antigenic properties and cause the formation of antibodies. In their composition, several types of antigenic determinants (epitypes) are distinguished: isotypes, allotypes and idiotypes.
Different antibodies differ from each other in their variable regions. The antigenic determinants of the variable regions (V regions) of antibodies are called idiotopes. Idiotopes can be constructed from characteristic sections of V-regions of only H-chains or L-chains. In most cases, both chains are involved in the formation of idiotope at once.
Idiotopes may be related to the antigen-binding site (site-associated idiotopes) or unrelated to it (non-associated idiotopes).
Site-associated idiotopes depend on the structure of the antigen-binding region of the antibody (belonging to the Fab fragment). If this site is occupied by an antigen, then the anti-idiotopic antibody can no longer react with an antibody that has this idiotope. Other idiotopes do not appear to have such close association with antigen-binding sites.
The set of idiotopes on the molecule of any antibody is designated as idiot. Thus, an idiotype consists of a set of idiotopes—antigenic determinants of the V region of an antibody.
Group constitutional variants of the antigenic structure of heavy chains are called allotypes. Allotypes are determinants encoded by alleles of a given immunoglobulin gene.
Isotypes are determinants that distinguish classes and subclasses of heavy chains and variants κ (kappa) and λ (lambda) of light chains.
Antibody affinity and avidity
The binding strength of antibodies can be characterized by immunochemical characteristics: avidity and affinity.
Under affinity understand the binding force between the active site of an antibody molecule and the corresponding antigen determinant. The strength of the chemical bond of one antigenic epitope with one of the active centers of the Ig molecule is called the binding affinity of the antibody to the antigen. Affinity is usually quantified by the dissociation constant (in mol-1) of one antigenic epitope with one active site.
Affinity is the accuracy of the coincidence of the spatial configuration of the active center (paratope) of the antibody and the antigenic determinant (epitope). The more connections are formed between the epitope and the paratope, the higher the stability and lifespan of the resulting immune complex will be. The immune complex formed by low-affinity antibodies is extremely unstable and has a short lifespan.
The affinity of antibodies for an antigen is called avidity antibodies. The avidity of the connection between an antibody and an antigen is the total strength and intensity of the connection between the entire antibody molecule and all the antigenic epitopes that it managed to bind.
Antibody avidity is characterized by the rate of formation of the antigen-antibody complex, the completeness of interaction and the strength of the resulting complex. Avidity, as well as the specificity of antibodies, is based on the primary structure of the determinant (active center) of the antibody and the associated degree of adaptation of the surface configuration of antibody polypeptides to the determinant (epitope) of the antigen.
Avidity is determined both by the affinity of the interaction between epitopes and paratopes, and by the valence of antibodies and antigen. Avidity depends on the number of antigen-binding centers in the antibody molecule and their ability to bind to numerous epitopes of a given antigen.
A typical IgG molecule, when both antigen-binding sites are involved, will bind to a multivalent antigen at least 10,000 times stronger than when only one site is involved.
Antibodies of class M have the greatest avidity, since they have 10 antigen-binding centers. If the affinities of the individual antigen-binding sites of IgG and IgM are the same, the IgM molecule (having 10 such sites) will exhibit incomparably greater avidity for the multivalent antigen than the IgG molecule (having 2 sites). Due to their high overall avidity, IgM antibodies, the main class of immunoglobulins produced early in the immune response, can function effectively even with low affinity of individual binding sites.
The difference in avidity is important because antibodies produced early in the immune response usually have much less affinity for the antigen than those produced later. The increase in the average affinity of antibodies produced over time after immunization is called affinity maturation.
Specificity of interaction between antigens and antibodies
In immunology, specificity refers to the selectivity of the interaction of inducers and products of immune processes, in particular, antigens and antibodies.
The specificity of interaction for antibodies is the ability of an immunoglobulin to react only with a specific antigen, namely, the ability to bind to a strictly defined antigenic determinant. The phenomenon of specificity is based on the presence of active centers in the antibody molecule that come into contact with the corresponding determinants of the antigen. The selectivity of the interaction is due to the complementarity between the structure of the active center of the antibody (paratope) and the structure of the antigenic determinant (epitope).
Antigen specificity is the ability of an antigen to induce an immune response to a strictly defined epitope. The specificity of an antigen is largely determined by the properties of its constituent epitopes.
One of the most important functions of immunoglobulins is antigen binding and the formation of immune complexes. Antibody proteins react specifically with antigens, forming immune complexes - complexes of antibodies associated with antigens. This connection is unstable: the resulting immune complex (IC) can easily disintegrate into its constituent components.
Each antigen molecule can be joined by several antibody molecules, since there are several antigenic determinants on the antigen and antibodies can be formed to each of them. As a result, complex molecular complexes arise.
The formation of immune complexes is an integral component of the normal immune response. The formation and biological activity of immune complexes depend, first of all, on the nature of the antibodies and antigen included in their composition, as well as on their ratio. The characteristics of immune complexes depend on the properties of antibodies (valence, affinity, rate of synthesis, ability to fix complement) and antigen (solubility, size, charge, valency, spatial distribution and epitope density).
Interaction of antigens and antibodies. Antigen-antibody reaction
The antigen-antibody reaction is the formation of a complex between an antigen and antibodies directed towards it. The study of such reactions is of great importance for understanding the mechanism of specific interaction of biological macromolecules and for elucidating the mechanism of serological reactions.
The effectiveness of the interaction of an antibody with an antigen significantly depends on the conditions under which the reaction occurs, primarily on the pH of the medium, osmotic density, salt composition and temperature of the medium. Optimal for the antigen-antibody reaction are the physiological conditions of the internal environment of the macroorganism: a close to neutral reaction of the environment, the presence of phosphate, carbonate, chloride and acetate ions, the osmolarity of the physiological solution (solution concentration 0.15 M), as well as a temperature of 36- 37 °C.
The interaction of an antigen molecule with an antibody or its active Fab fragment is accompanied by changes in the spatial structure of the antigen molecule.
Since no chemical bonds arise when an antigen is combined with an antibody, the strength of this connection is determined by the spatial accuracy (specificity) of the interacting sections of two molecules - the active center of the immunoglobulin and the antigenic determinant. The measure of bond strength is determined by the affinity of the antibody (the magnitude of the connection of one antigen-binding center with an individual epitope of the antigen) and its avidity (the total strength of interaction of the antibody with the antigen in the case of interaction of a polyvalent antibody with a polyvalent antigen).
All antigen-antibody reactions are reversible; the antigen-antibody complex can dissociate to release antibodies. In this case, the reverse antigen-antibody reaction proceeds much slower than the direct one.
There are two main ways by which an already formed antigen-antibody complex can be partially or completely separated. The first is the displacement of antibodies by an excess of antigen, and the second is the impact on the immune complex of external factors, leading to the severing of bonds (decreased affinity) between the antigen and the antibody. Partial dissociation of the antigen-antibody complex can generally be achieved by increasing the temperature.
When using serological methods, the most in a universal way dissociation of immune complexes formed by a wide variety of antibodies is their treatment with dilute acids and alkalis, as well as concentrated solutions amides (urea, guanidine hydrochloride).
Heterogeneity of antibodies
Antibodies formed during the body’s immune response are heterogeneous and differ from each other, i.e. They heterogeneous. Antibodies are heterogeneous in their physicochemical, biological properties and above all by its specificity. The main basis for the heterogeneity (diversity of specificities) of antibodies is the diversity of their active centers. The latter is associated with the variability of the amino acid composition in the V regions of the antibody molecule.
Antibodies are also heterogeneous in belonging to different classes and subclasses.
The heterogeneity of antibodies is also due to the fact that immunoglobulins contain 3 types of antigenic determinants: isotypic, characterizing the belonging of the immunoglobulin to a certain class; allotypic, corresponding to allelic variants of immunoglobulin; idiotic, reflective individual characteristics immunoglobulin. The idiotype-anti-idiotype system forms the basis of the so-called Jerne network theory.
Isotypes, allotypes, idiotypes of antibodies
Immunoglobulins contain three types of antigenic determinants: isotypic (the same for each representative of a given species), allotypic (determinants that are different among representatives of a given species) and idiotypic (determinants that determine the individuality of a given immunoglobulin and are different for antibodies of the same class or subclass).
In each biological species, the heavy and light chains of immunoglobulins have certain antigenic characteristics, according to which the heavy chains are divided into 5 classes (γ, μ, α, δ, ε), and the light chains into 2 types (κ and λ). These antigenic determinants are called isotypic (isotypes); for each chain they are the same in each representative of a given biological species.
At the same time, there are intraspecific differences in the named immunoglobulin chains - allotypes, determined by the genetic characteristics of the producing organism: their characteristics are genetically determined. For example, more than 20 allotypes have been described for heavy chains.
Even when antibodies to a particular antigen belong to the same class, subclass, or even allotype, they are characterized by specific differences from each other. These differences are called idiotypes. They characterize the “individuality” of a given immunoglobulin depending on the specificity of the inducer antigen. This depends on the structural features of the V-domains of the H- and L-chains and the many different variants of their amino acid sequences. All of these antigenic differences are determined using specific sera.
Classification of antibodies according to the reactions in which they can participate
Initially, antibodies were conventionally classified according to their functional properties into neutralizing, lysing and coagulating. Neutralizing agents included antitoxins, antienzymes and virus-neutralizing lysines. Coagulating agents include agglutinins and precipitins; to lysing - hemolytic and complement-fixing antibodies. Taking into account the functional ability of antibodies, names were given to serological reactions: agglutination, hemolysis, lysis, precipitation, etc.
Antibody studies. Phage display.
Until recently, the study of antibodies was difficult due to technical reasons. Immunoglobulins in the body are a complex mixture of proteins. The immunoglobulin fraction of blood serum is a mixture of a huge number of different antibodies. Moreover, the relative content of each type of them is, as a rule, very small. Until recently, obtaining pure antibodies from the immunoglobulin fraction was difficult to obtain. Difficulty in isolating individual immunoglobulins for a long time was an obstacle both to their biochemical research and to the establishment of their primary structure.
IN last years A new field of immunology has emerged - antibody engineering, which deals with the production of non-natural immunoglobulins with desired properties. For this purpose, two main directions are usually used: the biosynthesis of full-length antibodies and the production of minimal fragments of the antibody molecule, which are necessary for effective and specific binding to the antigen.
Modern technologies producing antibodies in vitro copy the selection strategies of the immune system. One such technology is phage display, which makes it possible to obtain fragments of human antibodies of different specificities. The genes from these fragments can be used to construct full-length antibodies.
In addition, very often therapeutic drugs created on the basis of antibodies do not require the involvement of their effector functions through the Fc domain, for example, in the inactivation of cytokines, blocking receptors or neutralizing viruses. Therefore, one of the trends in the design of recombinant antibodies is to reduce their size to a minimal fragment that retains both binding activity and specificity.
Such fragments in some cases may be more preferable due to their ability to penetrate tissue better and be eliminated from the body more quickly than full-length antibody molecules. At the same time, the desired fragment can be produced in E. coli or yeast, which significantly reduces its cost compared to antibodies obtained using mammalian cell cultures. In addition, this method of development allows one to avoid the biological hazard associated with the use of antibodies isolated from donor blood.
Myeloma immunoglobulins
Bence Jones protein. An example of a molecule of such an immunoglobulin, which is a dimer of kappa light chains
The term immunoglobulins refers not only to normal classes of antibodies, but also to a large number of abnormal proteins, commonly called myeloma proteins. These proteins are synthesized in large quantities in multiple myeloma, a malignant disease in which degenerated specific cells of the antibody-forming system produce large quantities of certain proteins, for example Bence-Jones proteins, myeloma globulins, fragments of immunoglobulins of various classes.
Bence Jones proteins are either single κ or λ chains or dimers of two identical chains linked by a single disulfide bond; they are excreted in the urine.
Myeloma globulins are found in high concentrations in the plasma of patients with multiple myeloma; their H and L chains have a unique sequence. At one time it was assumed that myeloma globulins are pathological immunoglobulins characteristic of the tumor in which they are formed, but now it is believed that each of them is one of the individual immunoglobulins, randomly “selected” from the many thousands of normal antibodies formed in the human body.
The complete amino acid sequence of several individual immunoglobulins has been determined, including myeloma globulins, Bence Jones proteins, and the light and heavy chains of the same myeloma immunoglobulin. Unlike the antibodies of a healthy person, all protein molecules of each named group have the same amino acid sequence and are one of many thousands of possible antibodies in an individual.
Hybridomas and monoclonal antibodies
Obtaining antibodies for human needs begins with immunizing animals. After several injections of the antigen (in the presence of immune response stimulants), specific antibodies accumulate in the blood serum of animals. Such sera are called immune sera. Antibodies are isolated from them using special methods.
However, the animal’s immune system produces special antibodies to a huge variety of antigens. This ability is based on the presence of a diversity of lymphocyte clones, each of which produces antibodies of the same type with narrow specificity. Total number clones in mice, for example, reaches 10^7 –10^10 degree.
Therefore, immune sera contain many antibody molecules with different specificities, i.e., having affinity for many antigenic determinants. Antibodies obtained from immune sera are directed both against the antigen that was immunized and against other antigens that the donor animal encountered.
For modern immunochemical analysis and clinical use, the specificity and standardization of the antibodies used are very important. It is necessary to obtain absolutely identical antibodies, which cannot be done using immune sera.
In 1975, J. Köhler and S. Milstein solved this problem by proposing a method for producing homogeneous antibodies. They developed the so-called “hybridoma technology” - a technique for producing cell hybrids (hybridoma). Using this method, hybrid cells are obtained that can multiply indefinitely and synthesize antibodies of narrow specificity - monoclonal antibodies.
To obtain monoclonal antibodies, plasmacytic tumor cells (plasmocytoma or multiple myeloma) are fused with the spleen cells of an immunized animal, most often a mouse. Köhler and Milstein's technology includes several stages.
Mice are injected with a specific antigen, which causes the production of antibodies against that antigen. Mouse spleens are removed and homogenized to obtain a cell suspension. This suspension contains B cells that produce antibodies against the administered antigen.
The spleen cells are then mixed with myeloma cells. These are tumor cells that are capable of continuously growing in culture; they also lack a reserve pathway for nucleotide synthesis. Some antibody-producing spleen cells and myeloma cells fuse to form hybrid cells. These hybrid cells are now able to grow continuously in culture and produce antibodies.
The mixture of cells is placed in a selective medium that allows only hybrid cells to grow. Unfused myeloma cells and B-lymphocytes die.
Hybrid cells proliferate, forming a hybridoma clone. Hybridomas are tested for production of the desired antibodies. Selected hybridomas are then cultured to produce large quantities of monoclonal antibodies that are free of extraneous antibodies and so homogeneous that they can be treated as pure chemical reagents.
It should be noted that antibodies produced by one hybridoma culture bind only to one antigenic determinant (epitope). In this regard, it is possible to obtain as many monoclonal antibodies to an antigen with several epitopes as it has antigenic determinants. It is also possible to select clones that produce antibodies of only one desired specificity.
The development of technology for producing hybridomas was of revolutionary importance in immunology, molecular biology and medicine. It made it possible to create completely new scientific directions. Thanks to hybridomas, new ways have opened up for the study and treatment of malignant tumors and many other diseases.
Currently, hybridomas have become the main source of monoclonal antibodies used in basic research and in biotechnology when creating test systems. Monoclonal antibodies are widely used in the diagnosis of infectious diseases of farm animals and humans.
Thanks to monoclonal antibodies, enzyme immunoassays, immunofluorescence reactions, flow cytometry methods, immunochromatography, and radioimmunoassays have become routine.
Many technologies have been developed to improve the synthesis of antibodies. These are DNA recombination technologies, cell cloning methods and other transgenic technologies. In the 90s, using genetic engineering methods, it was possible to minimize the percentage of mouse amino acid sequences in artificially synthesized antibodies. Thanks to this, in addition to mouse ones, chimeric, humanized and fully human antibodies were obtained.
CALL!Back in the 30s, it was shown that a protein molecule can bind several antibody molecules simultaneously.
In the 1950s, it became clear that antibodies interact with discrete sites on the surface of the protein molecule. They were called antigenic determinants. The problem was formulated: what constitutes an antigenic determinant? What properties allow a particular region of a protein to be recognized as foreign and trigger an immune response?
First, short synthetic peptides were used as a model. It turned out that linear homopolymers of amino acids (type (Ala-Ala) n) are non-immunogenic, but after conjugation with a carrier protein they behave like haptens, i.e. have antigen specificity. Polymer heteropolymers of amino acids are highly immunogenic and cause the synthesis of antibodies to the surface portions of the molecule. Peptides, taken in ordered or denatured form, had different antigenic specificity. If the synthetic nose antigen had charged groups, then the antibodies to it had the opposite charge.
It was concluded that antigenic determinants are located on the surface of the molecule, have a certain conformation and carry amino acid residues capable of forming non-covalent bonds with the antibody.
The main work on the antigenic structure of globular proteins was carried out in the 70-80s of the twentieth century. As a result, it was found that the antigenic determinant epitope is a separate region on the surface of a protein molecule. It consists of 6-7 amino acid residues. No connection was found with any specific amino acid residues: the antigenic determinants included those amino acids that are usually located on the surface of the protein. It turned out that each antigenic determinant describes a line 23-25 long on the surface of the protein. and has a deterministic N and C end.
There are sequential (linear) and discontinuous (conformational) antigenic determinants.
Sequential - determined by the order of amino acids. Antibodies to such epitopes easily interact with a linear peptide of the same sequence. In their pure form they are found in fibrillar proteins and peptides. In globular proteins, the surface successive regions have a specific conformation. Antibodies produced before peptides often recognize native proteins, i.e. can adapt in a certain way to the conformation of surface fragments.
Discontinuous antigenic determinants consist of amino acid residues located far from each other in the polypeptide chain, but brought together due to the tertiary structure of the protein, primarily disulfide bonds. Such antigenic determinants cannot be modeled by a linear peptide.
Not all amino acids that make up epitopes have the same importance for recognition: as a rule, specificity is determined by 1-2 residues (immunodominant), while others play a role in maintaining the proper conformation of epitopes.
As examples, consider the antigenic structure of sperm whale myoglobin and chicken egg lysozyme - the first protein antigens studied in detail.
Myoglobin is a heme protein of muscles with molecular weight 18 kDa, consisting of 153 amino acid residues, does not contain disulfide bonds. Five linear epitopes have been identified in the myoglobin molecule: fragments 16-21, 56-62, 94-99, 113-119 and 146-151. They included hydrophilic polar amino acids: Lys, Arg, Glu, His.
Lysozyme is an enzyme contained in the secretory fluids of the mammalian body and in the protein of bird eggs, with a molecular weight of 14 kDa, and has four disulfide bonds. Three discontinuous antigenic determinants were identified in the composition of lysozyme, which corresponded to fragments:
22-34 and 113-116, close disulfide bonds 30-115;
62-68 and 74-96, brought together by connections 76-94 and 64-80;
6-13 and 126-129, close connections 6-127.
To study these antigenic determinants, a special experimental approach, surface-mimicking synthesis, has been proposed. Thus, to simulate discontinuous epitopes, the residues were identified as immunodominant and stitched into a single peptide, combining individual fragments using a glycine spacer:
116 113 114 34 33
Lys Asn Arg Phe Lys
Lys-Asn-Arg-Gly-Phe-Lys
Such a peptide effectively blocked the binding of specific antibodies to the protein, i.e. was similar to the natural discontinuous epitope.
In the 1980s it became clear that the entire surface of a protein could be antigenic, i.e. If synthetic peptides are used for immunization, then antibodies can be obtained to any surface area. However, when immunized with the whole protein, antibodies were formed only to certain areas. The use of monoclonal antibodies of well-defined specificity has shown that each antigenic determinant is actually composed of several potentially overlapping antigenic sites. Now such epitopes have come to be called the more appropriate term immunodominant region.
Naturally, the question arose of what factors determine immunodominance.
Based on the recognized function of the immune system to distinguish “self” from “foreign”, the first principle underlying immunodominance was the principle of foreignness of the antigen in relation to the recipient proteins. To find out the validity of this principle, a series of homologous proteins was studied, i.e. proteins that are found in many organisms and differ in individual amino acid substitutions. Cytochromes c turned out to be ideal for such experiments.
Cytochromes c are heme proteins of the mitochondrial respiratory chain with a molecular weight of 13 kDa, consisting of about 100 amino acid residues. They appeared very early in the evolution of the living world; the first cytochromes c are found in bacteria. The protein structure turned out to be so successful that it was preserved in principle to higher animals. Mammalian cytochromes differ from each other in individual amino acid residues, i.e. can be considered as point mutants. A direct relationship was found between the immunogenicity of cytochrome c and the number of residues that distinguished the antigen from the homologous cytochrome c of the recipient. But regarding the specificity of the antibodies that were produced, this relationship did not turn out to be absolute. Thus, rabbits immunized with their own cytochrome modified glutaraldehyde
14
produced antibodies against epitopes of their own cytochrome. When animals different types immunized with one type of cytochrome, antibodies were produced against the same areas. Then they began to consider another principle of immunodominance - the connection with the structural features of the antigen: accessibility, charge, specific location on the fold of the subpeptide chain. Algorithms for searching for immunodominant sites were proposed based on the principles of hydrophilicity and atomic mobility. Further experiments revealed a connection between hydrophilicity and mobility and evolutionary variability: amino acid substitutions that were fixed in evolution should not disrupt biological functions cytochrome c and therefore were localized at the surface, most flexible areas, where the appearance of another amino acid is most safe and can be compensated for by the flexibility of the molecule.
As a result of these studies, it was concluded that although the entire surface of the protein can, in principle, be antigenic, during natural immunization with the native protein, antibodies are formed only to certain epitopes, the immunodominance of which is determined by their structural features, primarily hydrophilicity and atomic mobility (flexibility).
Antibodies (and B lymphocytes) bind the native antigen and recognize so-called B epitopes on its surface. But during the immune response, the antigen is also recognized by T lymphocytes. Moreover, it is the specificity of T lymphocytes that determines which immunodominant regions will be recognized as B epitopes. The regions of the antigen that are recognized by T lymphocytes are called T epitopes. Their position and structure are not determined as easily as for B epitopes, because T cells recognize antigens in a completely different way.
1. For recognition by T lymphocytes, the antigen must be processed (split). Processing occurs inside specialized cells under the action of proteolytic enzymes. The spectrum of peptides produced depends on the type of proteases, which differ in different cell types.
2. The processing peptide must be presented in complex with the proteins of the major histocompatibility complex: the selection of the antigenic peptide depends on the structure of these proteins, which are highly polymorphic and differ even in different individuals of the same species.
3. Recognition of the presented peptide depends on the T-cell receptor repertoire, which is the result of positive and negative selection in a particular individual.
As a result, the T epitope is not necessarily a surface structure; not conformation-dependent, but a linear peptide. Its position is not related to the hydrophilicity or mobility of the polypeptide chain. It depends both on the structure of the native protein (potential proteolysis sites, peptide motifs corresponding to the binding sites of histocompatibility proteins) and on the state of the individual recipient’s immune system (repertoire of histocompatibility proteins and T-cell receptors). T epitopes are more associated with sites of antigen foreignness to recipient proteins than B epitopes, since the T receptor repertoire undergoes more stringent negative selection.
Determining the structure and localization of B and T epitopes is not only of fundamental interest. It is necessary for the creation of effective vaccines and immunodiagnostics.
The immune system is capable of recognizing almost any substance from the environment surrounding the macroorganism. For this to happen, the antigen must be properly presented to immune cells. Lymphocytes and antibodies recognize conformation-dependent surface epitopes located in places of greatest hydrophilicity and flexibility of the polypeptide chain. T lymphocytes recognize internal linear peptide fragments that are formed as a result of proteolysis (processing) of the native antigen.