Characteristics of the main classes of immunoglobulins.

Basic biological characteristics of antibodies.

1. Specificity- the ability to interact with a specific (own) antigen (correspondence of the antigen epitope and the active center of antibodies).

2 . Valence- the number of active centers capable of reacting with the antigen (this is due to the molecular organization - mono- or polymer). Immunoglobulins can be bivalent(IgG) or polyvalent(IgM pentamer has 10 active centers). Bivalent or more valence antibodies are applied complete antibodies. Incomplete antibodies have only one active center involved in the interaction with the antigen (blocking effect on immunological reactions, for example, on agglutination tests). They are detected in the Coombs' antiglobulin test, the reaction of inhibition of complement binding.

3. Affinity - the strength of the bond between the antigen epitope and the active center of antibodies depends on their spatial correspondence.

4. Avidity - integral characteristic of the strength of the bond between antigen and antibodies, taking into account the interaction of all active centers of antibodies with epitopes. Since antigens are often polyvalent, communication between individual antigen molecules is carried out using several antibodies.

5. Heterogeneity - due to the antigenic properties of antibodies, the presence of three types of antigenic determinants in them:

- isotypic- belonging of antibodies to a certain class of immunoglobulins;

- allotypical due to allelic differences in immunoglobulins encoded by the corresponding alleles of the Ig gene;

- idiotypic- reflect the individual characteristics of immunoglobulin, determined by the characteristics of the active centers of antibody molecules. Even when antibodies to a specific antigen are related to

one class, subclass and even allotype, they are characterized by specific differences from each other ( idiotype). It depends on the structural features of the V-regions of the H- and L-chains, many different variants of their amino acid sequences.

The concept of polyclonal and monoclonal antibodies will be given in the following sections.

Ig G. Monomers include four subclasses. The concentration in the blood is from 8 to 17 g / l, the half-life is about 3-4 weeks. It is the main class of immunoglobulins that protect the body from bacteria, toxins, and viruses. V the greatest number IgG antibodies are produced at the stage of recovery after an infectious disease (late or 7S antibodies), with a secondary immune response. IgG1 and IgG4 specifically (via Fab fragments) bind pathogens ( opsonization), thanks to Fc fragments, IgG interact with Fc receptors of phagocytes, promoting phagocytosis and lysis of microorganisms. IgG is able to neutralize bacterial exotoxins, bind complement. Only IgG is able to be transported across the placenta from mother to fetus (pass through the placental barrier) and provide maternal antibodies protection to the fetus and newborn. Unlike IgM antibodies, IgG antibodies belong to the category of late ones - they appear later and are detected in the blood for a longer time.

IgM. The molecule of this immunoglobulin is a polymeric Ig of five subunits connected by disulfide bonds and an additional J-chain, has 10 antigen-binding sites. Phylogenetically, this is the most ancient immunoglobulin. IgM is the earliest class of antibodies formed when the antigen first enters the body. The presence of IgM antibodies to the corresponding pathogen indicates a fresh infection (current infectious process). IgM is the main class of immunoglobulins synthesized in newborns and infants. IgM in newborns is an indicator of intrauterine infection (rubella, CMV, toxoplasmosis and other intrauterine infections), since maternal IgM does not pass through the placenta. The concentration of IgM in the blood is lower than IgG - 0.5-2.0 g / l, half-life is about a week. IgM are able to agglutinate bacteria, neutralize viruses, activate complement, activate phagocytosis, bind endotoxins of gram-negative bacteria. IgM have greater avidity than IgG (10 active sites), affinity (affinity for the antigen) is less than that of IgG.

IgA. Serum IgA (monomer) and secretory IgA (IgAs) are isolated. Serum IgA is 1.4-4.2 g / l. Secretory IgAs are found in saliva, digestive juices, nasal secretions, and colostrum. They are the first line of defense of the mucous membranes, providing them with local immunity. IgAs are composed of an Ig monomer, a J chain, and a glycoprotein (secretory component). There are two isotypes - IgA1 predominates in serum, IgA2 subclass - in extravascular secretions.

The secretory component is produced by the epithelial cells of the mucous membranes and attaches to the IgA molecule at the moment the latter passes through the epithelial cells. The secretory component increases

resistance of IgAs molecules to the action of proteolytic enzymes. The main role of IgA is to provide local mucosal immunity. They prevent bacteria from attaching to mucous membranes, provide transport of polymeric immune complexes with IgA, neutralize enterotoxin, activate phagocytosis and the complement system.

IgE... It is a monomer and is found in low concentrations in blood serum. The main role - with its Fc-fragments, attaches to mast cells (mast cells) and basophils and mediates immediate hypersensitivity reactions. IgE includes “allergy antibodies” - reagin. IgE level rises in allergic conditions, helminthiasis. Antigen-binding Fab-fragments of the IgE molecule specifically interacts with the antigen (allergen), the formed immune complex interacts with the receptors of the Fc-fragments of IgE embedded in the cell membrane of the basophil or mast cell. This is a signal for the release of histamine, other biologically active substances and the deployment of an acute allergic reaction.

IgD. IgD monomers are found on the surface of developing B-lymphocytes and are found in extremely low concentrations in serum. Their biological role has not been precisely established. It is believed that IgD are involved in the differentiation of B cells, contribute to the development of an anti-idiotypic response, and are involved in autoimmune processes.

In order to determine the concentrations of immunoglobulins of certain classes, several methods are used, more often they use method of radial immunodiffusion in gel (according to Mancini) - a kind of precipitation reaction and ELISA.

Determination of antibodies of various classes is essential for diagnosis infectious diseases... Detection of antibodies to antigens of microorganisms in blood serum is an important criterion for making a diagnosis. serological method diagnostics. Antibodies of the IgM class appear in the acute period of the disease and disappear relatively quickly, antibodies of the IgG class are detected in more late dates and for a longer time (sometimes for years) are retained in the blood serum of those who have been ill, in this case they are called anamnestic antibodies.

Highlight concepts: antibody titer, diagnostic titer, studies of paired sera. Of greatest importance is the detection of IgM antibodies and a fourfold increase in antibody titers (or seroconversion- antibodies are detected in the second sample with negative results with the first blood serum) in the study paired- samples taken in the dynamics of the infectious process with an interval of several days or weeks.

Reactions of interaction of antibodies with pathogens and their antigens ( antigen-antibody reaction) manifests itself in the form of a number of phenomena - agglutination, precipitation, neutralization, lysis, complement binding, opsonization, cytotoxicity and can be identified by various serological reactions.

The primary response is upon primary contact with the pathogen (antigen), the secondary response is upon repeated contact. The main differences are:

The duration of the latent period (more for the primary);

The rate of rise of antibodies (faster - with secondary);

The amount of synthesized antibodies (more - with repeated contact);

The sequence of synthesis of antibodies of various classes (with the primary IgM predominate for a longer time, with the secondary, IgG antibodies are rapidly synthesized and predominate).

The secondary immune response is due to the formation immune memory cells. An example of a secondary immune response is a meeting with a pathogen after vaccination.

The dynamics of the formation of antibodies has a different character depending on the strength of the antigenic effect (dose of antigen), the frequency of exposure to the antigen and its immune system. Antibody formation takes place in several stages:

1) latent phase - processing and presentation of antigen to immunocompetent cells and multiplication of a clone of plasma cells occurs. Antibody synthesis begins. During this period, antibodies are not detected in the blood;

2) logarithmic phase - synthesized antibodies are released from plasma cells and enter the lymph and blood;

3) stationary phase - the amount of antibodies reaches a maximum and stabilizes;

4) the phase of decreasing the level of antibodies.

With the primary administration of antigen (primary immune response), the latent phase is 3-5 days, the stationary phase is 15-30 days, the decline phase is 1-6 months or more.

A feature of the primary immune response is that it is initially synthesized by IgM, then IgG, and later IgA.

The main differences between the secondary response and the primary response are as follows:

Shorter latency period (up to several hours or 1-2 days); a faster rise and a higher level of antibody concentration (the maximum concentration increases by 3 times); a slow decline in antibody levels, sometimes over several years; mainly IgG is synthesized.

This difference in antibody production in the primary and secondary immune response is explained by the fact that after the primary administration of the antigen, a clone of lymphocytes with the immunological memory of this antigen is formed in the immune system. After a repeated encounter with the same antigen, a clone of lymphocytes with immunological memory multiplies rapidly and intensively turns on the process of antitelogenesis.

Very rapid vigorous antibody formation upon repeated encounter with the antigen is used to obtain high antibody titers in the production of diagnostic and therapeutic sera from immunized animals, as well as for the urgent creation of immunity during vaccination.

18. CHARACTERISTICS OF HUMORAL AND CELL IMMUNE RESPONSE.

It is customary to distinguish following forms immune response: 1) humoral response, 2) cellular response, 3) immediate-type hypersensitivity, 4) delayed-type hypersensitivity; 5) immunological memory; 6) immunological tolerance.

Immune response occurs as a result interactions Agroindustrial complex (dendritic cells, macrophages), T- and B-lymphocytes, cytokines. It includes: 1) antigen recognition; 2) activation of cells; 3) their differentiation and proliferation.

Cells interact: 1) upon contact through special receptors on the cell membrane; 2) using cytokines .

Humoral immune response (antibody production)... The basis of the humoral immune response is the activation of B-lymphocytes and their differentiation into antibody-forming plasma cells - plasma cells.

It involves B-lymphocytes and T H 2-helpers.

B-lymphocytes play a role antigen-presenting and antibody-forming cells.

T H 2-helpers differentiate from T H 0-helpers (naive, null) after recognition of the antigen-MHC class II complex on antigen-presenting cells (APC, for example, macrophages).

Macrophage presentation this complex T H 0-helpers includes:

1) absorption of antigen and its cleavage (processing) to antigenic peptides;

2) binding of antigenic peptides to MHC class II molecules formed inside the cell (“loading” MHC molecules into the grooves);

3) the release of the antigen-MHC class II complex on the cell surface for contact with the TCR T H 0-helper.

When the antigen is presented, immune synapse - zone(a place) contact between cells for antigen recognition and signal transmission into the cell. Includes: TCR(on T H 0) + antigen - MHC class II(on macrophage) + coreceptor CD4(on T H 0) . Thus, TCR recognizes the changed "self", realizing double recognition of "friend" from "alien". In this case, the TCR of one lymphocyte is recognized only one antigen. T H 0-helper turns into T H 2- helper.

Thereafter, T H 2 helpers interact with B-lymphocytes. The B-lymphocyte recognizes the antigen using the BCR (immunoglobulin receptor) and the cell absorbs it. After cleavage of the antigen to a low molecular weight peptide ( processing) and integrating it into the MHC class II, the B-lymphocyte presents a complex of antigen-MHC class II to the T H 2 helper, which interacts with it using the TCR and the CD4 coreceptor. Immune synapse includes: TCR(on T H 2) + antigen - MHC class II(on B-lymphocyte) + coreceptor CD4(on T H 2). Further, a CD40 ligand appears on the surface of the T H 2 helper, which binds to the CD40 receptor on the B lymphocyte. After that, proliferation starts, the differentiation of cells into plasma cells that synthesize immunoglobulins of various classes. The proliferation of B-lymphocytes is enhanced by IL-3. Interleukins (IL-4, IL-5, IL-6, IL-10, IL-13) produced by T H 2 are involved in switching the synthesis of classes of immunoglobulins. Plasma cells synthesize antibodies of the same specificity.

The formed antibodies specifically bind to the antigens that caused their formation - they are formed antigen-antibody complexes... Antigen-antibody complexes are destroyed by complement (due to the formation of MAC) or absorbed and digested by macrophages (immune phagocytosis).

There can be many different antigens on the surface of a single microbe, so a series of antibodies are usually produced, each of which is directed at a specific antigen.

Cellular immune response- formation of a clone of cytotoxic T-lymphocytes - CTLs (CD8), capable of destroying target cells, whose membranes contain foreign materials (for example, viral proteins).

The cellular immune response underlies antitumor, antiviral immunity and transplant rejection reactions, i.e. transplant immunity.

Involved in the cellular immune response T H 1-helpers, CTL and APC. Antigen presenting cells - APGs (macrophages and dendritic cells) absorb antigen and, after processing, represent:

1) antigen-MHC class I complex ® CTL; immune synapse includes: TCR(at CTL) + antigen - MHC class I(on macrophage) + coreceptor SD8(at the CTL);

2) class II antigen-MHC complex ® T H 0; immune synapse includes: TCR(on T H 0) + antigen-MHC class II(on macrophage) + coreceptor CD4(on T H 0) (as in the humoral immune response, but at the same time T H 0 ® T H 1).

Thus, with the help of the TCR and the CD8 coreceptor, CTL recognizes the antigen and MHC class I (double recognition), and T H 0 using the TCR and the CD4 coreceptor recognizes the antigen and MHC class II and differentiates into T H 1. The T H 1 secretes IL-2, under the influence of which the proliferation of CTL occurs. Then CTLs “recognize” target cells infected with intracellular microbes (for example, viruses). The target cells display microbial antigens in combination with MHC class I, which are recognized by the TCR and the CD8 coreceptor. Activated and differentiated CTLs cause the death of target cells with the help of their secreted cytotoxic proteins: perforins, granulisins, granzymes, which, embedding in the membrane of the target cell, form pores that facilitate the penetration of granzymes that trigger apoptosis target cells.

A type of cellular immune response is delayed-type hypersensitivity (HRT) with T H 1 helper cells and activated macrophages. The greatest role in the activation of macrophages and NK cells is played by γ-interferon secreted by T H 1. Activated macrophages produce effective destruction of the antigen.

Immunoglobulins are glycoprotein molecules that are produced by plasma cells in response to an immunogen-antigen (a foreign molecule that includes an immune response - surface molecules of bacteria, viruses, fungi). Immunoglobulins function as antibodies.

General functions of immunoglobulins:

• Specific antigen binding -protective function

• Activation complement,
• Communication with various cells of the immune system

General structure of immunoglobulins (Fig. 1).

Immunoglobulins (Igs) are glycoproteins composed of light (L) and heavy (H) polypeptide chains.
The simplest antibody molecule has the Y form and consists of four polypeptide chains: two H chains and two L chains. Four chains are linked by disulfide bridges. In the antibody molecule, variable (V L and V H) and constant (C L and C H) regions and a hinge region are distinguished.

H-chains are different for each of the five classes (isotypes) of immunoglobulins and are denoted by γ, α, μ, δ and ε, The type of heavy chain determines the name of the class, namely
IgA, IgG, IgM, IgD, IgE. There are only two types of light chains κ and λ. In structure immunoglobulin molecules contain only one of two types of light chains.

The L and H chains are subdivided into variable and constant regions. Regions are made up of three-dimensionally stacked, repeating segments called domains. The L circuit consists of one variable (V L) and one constant (C L) domain. Most of the H chains consist of one variable (V H) and three constant (C H) domains (IgG and IgA have three C H domains, while IgM and IgE have four.

Variable regions are carried responsible for antigen binding, whereas constant- are responsible for various biological functions, for example, complement activation, binding to cell surface receptors, transfer across the placenta ..

Both variable regions of the L and H-chains have three extremely variable ("hypervariable") amino acid sequences at the N end. They form the antigen binding site.

Under the action of a proteolytic enzyme, ne psin, immunoglobulin molecules are split into two fragments: F (ab) 2 - binding antigen, and Fc - crystallizing. Fc domains perform biological, effector functions of immunoglobulins.

With electropho cutaneous serum immunoglobulins migrate in the fraction of gamma globulins. Teats on gamma globulins is used to estimate the amount of immunoglobulins in the blood.Immunoglobulins are produced by the body in response to foreign substances such as bacteria, viruses, and cancer cells.

A gamma globulin test is a diagnostic procedure that can help doctors identify a problem in order to begin treatment.It should be noted that this test is only performed in the case of serious medical conditions.

The results of the determination of immunoglobulins are issued after a few days, normal values are as follows:

• IgA: 85 - 385 mg / dL
• IgG: 565 - 1765 mg / dL
• IgM: 55 - 375 mg / dL
• IgD: 8 mg / dL or less
• IgE: 4.2 - 592 mg / dl

Evaluation of the results of the analysis for immunoglobulins (antibodies)

High and low values ​​are not normal and may be a sign of an underlying medical condition.

High IgA values may be a sign of multiple myeloma, liver cirrhosis, chronic hepatitis, rheumatoid arthritis and systemic lupus erythematosus or SLE.

Low IgA values may be a sign of kidney damage, some types of leukemia and enteropathy.

High IgG values can be a sign of AIDS, multiple sclerosis, and chronic hepatitis.

Low IgG values can be a sign of macroglobulinemia, nephrotic syndrome and some types of leukemia.

Low IgM values may indicate multiple myeloma, some types of leukemia, and hereditary immune diseases.

Low IgE values are indicative of a disease called ataxia-telangiectasia. This is a rare condition in which muscle function is impaired.

Gamma Globulin Therapy

During electrophoresis of blood serum proteins on paper or agar, due to the different molecular weight / charge ratio, proteins move from different speed... As a result, fractions of albumin, alpha, beta and gamma globulins are formed. The gamma globulin fraction is represented by antibodies, the aggregate of which is called gamma globulin.

It has been proven that gamma globulin from human blood can be used to treat infections. This method is called gamma globulin therapy. The procedure involves injecting a gamma globulin preparation into a vein or muscle.

The dynamics of antibody production in response to antigenic stimulation is largely determined by the species belonging of the individual, since it is genetically determined (Vershigora A.V., 1990). Nevertheless, general patterns of antibody production were found, which are characteristic of various species of animals and humans. The latter are as follows.

The intensity of antibody production depends on the structural features of the antigen, the method of administration of the antigen and the way of its penetration into the body.

The production of antibodies depends on the state of the body's immunological reactivity, which is determined, in turn, by the level of representativeness of that clone of lymphocytes that is capable of receiving this antigen, the presence or absence of mutations of this clone that can affect the quantity and quality of synthesized immuno-globulins.

The nature of the immune response is undoubtedly determined by the functional activity of macrophage elements, including various populations of classical phagocytes with a less pronounced ability to present antigen in the reactions of the primary immune response, as well as antigen-presenting macrophages with a slightly pronounced phagocytic activity.

The intensity of antibody production depends on the hormonal status, functional activity of the central nervous system... Excess hormonal background generated by ACTH, glucocorticoids, as well as the lack of insulin can adversely affect the processes of anti-body formation.

The strength of the immune response also depends on the general state of the body, the duration of previous diseases of an infectious and non-infectious nature, the nature of the effect of stress stimuli, the state of the electrolyte balance of the body, the acid-base state, the degree of intensification of free radical lipid oxidation in biological membranes.

It is well known that with the development of various typical pathological processes, nonspecific destabilization of biological membranes of cells of various organs and tissues, swelling of mitochondria, ATP deficiency, suppression of all energy-dependent reactions in cells, including the synthesis of antibodies of various classes of immunoglobulins, occur.

It has been established that immunization of a person with antigens of a protein, viral nature, lipopolysaccharide antigens of enterobacteria stimulates the formation of antibodies mainly of the IgG class, and in guinea pigs, such antigens mainly enhance the synthesis of antibodies of the IgM class. A relatively large number of antibodies are synthesized per one molecule of the injected antigen. Thus, for each molecule of injected diphtheria ana-toxin, over a million antitoxin molecules are synthesized within 3 weeks.

For each antigen, there are optimal doses of influence on the immune system. Small doses induce a weak response, extremely large doses can cause the development of immunological tolerance or have a toxic effect on the body.

During primary antigenic exposure, 4 phases of the immune response develop.

1st phase of antibody production

1st phase of antibody production (resting phase, lag phase, induction phase, or latent phase), that is, the period between the time the antigen enters the body and before the start of the exponential increase in antibodies (Yeger L., 1986; Led- Vanov M.Yu., Kirichuk V.F., 1990).

The duration of this phase can be different depending on the nature of the antigen: from several minutes and hours to a month.

The essence of this phase lies in the development of macrophage reaction, phagocytosis or endocytosis of the antigen by antigen-presenting or phagocytic macrophages, in the formation of highly immunogenic antigen fractions in combination with MHC class I and II antigens, antigen presentation to B- and T-lymphocytes, and cooperative interactions -precise elements and antigen-sensitive subpopulations of T- and B-lymphocytes, the development of plasmatization of lymphoid tissue. As mentioned above, one of the features of lymphoid cells is the preservation in them of a unique chromosome-repairing enzyme of the hematopoietic stem cell - telomerase, which provides the possibility of repeated cyclic proliferation during life against the background of antigenic stimulation.

As you know, there are two mechanisms of activation of resting B-lymphocytes with their subsequent inclusion in proliferation and differentiation.

For the main subpopulation of B2-lymphocytes, differentiating in the bone marrow, inclusion in the immune response is provided by their interaction with T-helper cells, restricted by the main histose-locus complex, as well as various cytokines - growth and proliferation factors.

The selected clone of B-lymphocytes enters the proliferation phase, which provides an increase in the representation in the lymphoid tissue of the antigen-sensitive clone of B-lymphocytes capable of further transformation.

BI (CD5) subpopulation of lymphocytes leaving Bone marrow in the early period of embryonic development and differentiating outside the bone marrow, it is capable of T-independent activation under the influence of a certain group of antigens - bacterial polysaccharides. In the process of plasmatization of the ВI-subpopulation of lymphocytes against the background of antigenic stimulation, class M immunoglobulins with a wide cross-reactivity are formed.

2nd phase of antibody production

2nd phase of antibody production (logarithmic phase, log phase, productive phase). This phase is called the phase of exponential growth of antibodies. It takes a period of time from the appearance of antibodies to reaching the maximum amount of them in the blood, on average it lasts from 2 to 4 days. In some cases, the duration of the phase increases to 15 days.

An exponential increase in the number of antibodies, a doubling of their titers, occurs initially every 2-4 hours, and then every 4-6 hours. However, the rate of antibody production by the end of the second or third day slows down, remaining at a certain level for a different period of time.

3rd phase of antibody production

The 3rd phase of antibody production is the stabilization phase, or the stationary period, during which the antibody titer remains stably high. During this period, the transition of cells from the class of activated precursors to the class of antibody-forming cells stops.

The duration of the stabilization phase is largely determined by the structural features of antigens-allergens. In some cases, it continues for several days, weeks, months. Antibodies to some microbial antigens continue to be synthesized in a fairly high titer for a number of years.

Regarding the significance of this stabilization phase, it should be noted that antibodies not only provide inactivation of bacterial, toxic, allergic pathogenic factors in various reactions of agglutination, precipitation, complement activation, antibody-dependent cytolysis, but also act as autoregulators of immunopoiesis.

4th phase of decrease in antibody production

The duration of this phase is different and depends on the preservation of the antigen in the tissues.

The above-described dynamics of antibody production occurs in the case of primary immunization. Re-immunization after several months changes the dynamics of the immune response. The latent period and the period of increase in the antibody titer become much shorter, the amount of antibodies reaches a maximum faster and remains at a high level longer, and the affinity of antibodies increases.

An important role in the development of the secondary immune response is assigned to an increase in the level of immunological memory cells to this antigen. With an increase in the duration of immunization, the specificity of antibodies to soluble antigens increases.

It should be noted that the formation of antigen-antibody complexes during multiple immunization increases the strength of the antigenic effect and the intensity of antibody production.

It has been established over the past decades that the synthesis of immunoglobulins is a self-regulating process. Proof of this is the inhibitory effect on the production of antibodies of specific immunoglobulins introduced into the bloodstream, and the higher the affinity of antibodies, the more intense their inhibitory effect on the processes of immuno-poetry. Antibodies can have an inhibitory effect on the synthesis of not only homologous, but also related immunoglobulins. The formation of antibodies can also be inhibited by large doses of nonspecific β-globulins.

Structure and functional significance of immunoglobulins.

Proteins belonging to the family of immunoglobulins have the same structural principle: their molecules include light and heavy polypeptide chains (Dolgikh R.T., 1998).

According to the WHO nomenclature (1964), there are 5 classes of immunoglobulins: IgG, IgA, IgM, IgE, IgD. Each class of immunoglobulins is characterized by its own specific heavy H-chains, designated according to the class of immunoglobulins (m, g, a, d, e). It is the peculiarities of the structure of the H-chains that determine the belonging of the immunoglobulin to one or another class.

Immunoglobulins are formed by at least four polypeptide chains interconnected by disulfide bridges. Two of them are represented by heavy H-chains, and two by light L-chains. There are two types of light chains k and l, which can be found in immunoglobulins of each of the 5 classes. Immunoglobulins of classes G, D and E are monomers, at the same time IgM occurs mainly in the form of a pentamer, and IgA - in the form of mono-, di- and tetramer. Polymerization of monomers in molecules of immunoglobulins of classes A and M is provided by the presence of additional J-chains (Vershigora A.V., 1990; Royt A., 1991; Stephanie D.F., Veltischev Yu.E., 1996).

In both heavy and light chains, there is a variable V region, in which the amino acid sequence is not constant, as well as a constant, constant, C-region.

Variable regions of light and heavy chains take part in the formation of the active center of antibodies, determine the specificity of the structure of the antideterminant of antibodies, which ensures the binding of the determinant of the antigen.

One antibody molecule can have unambiguous light chains (k or l).

Antibodies of different specificity can be contained in any of the classes of immunoglobulins. In lymphoid tissue, in response to the action of the same antigen, the synthesis of polypeptide chains of different classes of immunoglobulins occurs simultaneously.

Common in the structure of immunoglobulins of various classes is the presence of the so-called Fab-fragments (Fragment antigen binding), Fc-fragment (Fragment crystalline) and Fd-fragment (Fragment difficult).

The Fab fragment includes antigen-sensitive receptor groups capable of specifically binding an antigen. The formation of the Fab-fragment involves the CD-region (amino-terminal part of the heavy chain), and, possibly, a fragment of the variable part of the light chain.

The Fc fragment determines the nonspecific functions of antibodies: fixation of complement, the ability to cross the placenta, fixation of immunoglobulins on cells.

The study of the structure of immunoglobulins is difficult due to their heterogeneity. The heterogeneity of immunoglobulins is due to the fact that immunoglobulin molecules are carriers of various sets of determinants. There are three main types of antibody heterogeneity: isotype, allotype, idiotype.

Isotypic variants of antibodies are found in all individuals. These include subclasses of different types of immunoglobulins.

In the IgG class, 4 isotypes are known (IgG1, IgG2, IgG3, IgG4), in the IgA, IgM and IgD classes there are 2 isotypes, or subclasses.

The isotypic determinants of antibodies of the same class and subclass are identical in individuals of this species. Isotypic differences are determined by the amino acid sequence in the constant part of the heavy chains, as well as the number and position of disulfide bridges. So, IgG1 and IgG4 have four interchain disulfide bonds, two of which connect the H-chains. The IgG2 molecule has six disulfide bridges, four of which bind polypeptide chains.

Isotypic variants include k and l - types and subtypes of L-chains.

Variable regions of light chains of a certain type can be divided into subgroups. L-chains of k-type have 4 subgroups, L-chains have l 5 subgroups. Chains of different subgroups, in addition to differences in the primary structure, are characterized by a variation in the sequence of twenty N-terminal amino acids.

For the variable portion of the H chain, 4 subgroups are described.

Allotype variants of immunoglobulins in humans and animals are genetically determined, their frequency varies in individuals different types... Allotypes are allelic variants of polypeptide chains that occur during mutations. Allotype synthesis is controlled by various gene alleles. There are six rabbit globulin allotypes. Currently, many systems of allotypic markers of human immunoglobulins are known, located in the C-region of L and H-chains. The existence of some of these markers is due to the development of a point mutation and replacement of only one amino acid in the polypeptide sequence. If the mutation affects the structure of a region specific to a certain class and subclass of immunoglobulins, an allotypic variant is formed.

Several allotypic markers can be found in the serum of one individual.

Idiotypic differences in antibodies essentially reflect the specificity of antibodies. They are associated with variable regions of polypeptide chains, do not depend on the structural features of different classes of immunoglobulins, and are identical in different individuals if they have antibodies to the same antigen.

There are about the same number of idiotypic variants as there are antibodies of different specificity. The belonging of an antibody to a certain idiotype of immunoglobulins determines the specificity of its interaction with the antigen. It is generally accepted that the presence of 5,000 to 10,000 different variants of antibody specificity is sufficient to associate with greater or lesser affinity any of the possible varieties of antigenic determinants. Currently, antigenic determinants of the V regions are also called idiotypes.

Affinity and avidity are essential properties antibodies of different classes of immunoglobulins, and the affinity reflects the strength of the bond of the active center of antibodies with the antigen determinant, while avidity characterizes the degree of antigen binding by the antibody, determined by the affinity and the number of active sites of the antibody.

A heterogeneous population of antibodies has a set of antideterminants with different affinity, therefore, when determining its avidity, we determine the average affinity. With equal affinity, the avidity of IgM can be greater than that of IgG, since IgM functionally has five valences, and IgG is bivalent.

Genetics of antibody formation

As mentioned above, immunoglobulins of various classes and subclasses are represented by heavy and light polypeptide chains, each of which has variable and constant regions. It has now been established that the synthesis of the variable region is under the control of many V-genes, the number of which is approximately equal to 200.

In contrast, a limited number of C-genes is known for the constant region in accordance with its insignificant variability (class, subclass, type, subtype).

On initial stages the formation of lymphoid tissue V- and C-genes are located in far-apart DNA segments, and in the genome of maturing immunocompetent cells, they are combined due to translocation in one sublocus, which controls the synthesis of H- and L-chains.

The formation of a variety of antibodies is explained by the hypothesis of somatic hypermutability of V-genes, which is unlikely, as well as by hypotheses of genetic recombination of genes and recombination errors.

General characteristics of certain classes of immunoglobulins

Due to the peculiarities of the physicochemical structure, antigenicity and biological functions, there are 5 main classes of immunoglobulins (IgM, IgG, IgA, IgE, IgD).

It should be noted that antibodies of the same specificity can belong to different classes of immunoglobulins; at the same time, antibodies of different specificity can belong to the same class of immunoglobulins.

Immunoglobulins of class M

Immunoglobulins of class M are the earliest in both phylogenetic and ontogenetic terms. In the embryonic period and in newborns, IgM is synthesized mainly. IgM accounts for about 10% of the total amount of immunoglobulins, their average concentration in the serum of women is 1.1 g / l, in the serum of men - 0.9 g / l.

Antibodies of the IgM class are pentavalent, have a pronounced ability to agglutinate, precipitate and lyse antigens. Of all types of antibodies, IgM exhibit the highest complement binding capacity. IgM are found mainly in blood plasma and lymph, the rate of their biosynthesis is about 7 mg / day, the half-life is 5.1 days. IgM does not cross the placenta. Detection of IgM in the fetus in a high concentration indicates an intrauterine infection.

Regarding the structural organization of IgM, it should be noted that IgM molecules have a MW of 900,000 with a sedimentation constant of 19S, include 5 subunits connected by disulfide bonds between heavy chains. Each IgM subunit has a MW of 180,000 and a sedimentation constant of 7S, which is structurally identical to an IgG molecule.

By acting on the IgM molecule with pepsin, trypsin, chymotrypsin, pa-pain, various fragments (Fab, Fd, Fc) can be obtained. IgM contains a J-chain that participates in the polymerization of the molecule.

Depending on the ability to fix complement with the participation of the Fc-fragment, IgM are divided into two subclasses: IgM1 and IgM2. IgM1 bind complement, IgM2 do not bind complement.

In an electrophoretic study, macroglobulins migrate in the zone of the -globulin fraction.

By the end of the 2nd year of a child's life, the IgM content is 80% of its content in adults. The maximum concentration of IgM is observed at 8 years.

Class G immunoglobulins

IgG are the most studied class of immunoglobulins, they are contained in the blood serum in the highest concentration compared to other immunoglobulins (on average 12.0 g / l), accounting for 70-75% of the total amount of immunoglobulins.

The molecular weight of IgG is 150,000, the sedimentation constant is 7S.

Possessing two antigen-binding centers, IgG form a network structure with multivalent antigens, cause precipitation of soluble antigens, as well as agglutination and lysis of corpuscular and pathogenic agents.

There are 4 IgG subclasses: IgG1, IgG2, IgG3, IgG4.

The subclasses IgG3, IgG1 and IgG2 have the maximum ability to activate complement by the classical pathway. The IgG4 subclass is capable of activating complement via an alternative pathway.

Antibodies belonging to the subclasses IgG1, IgG3, IgG4 freely penetrate the placenta, antibodies of the IgG2 subclass have a limited ability of transplacental transport.

IgGs form the main line of specific immunological defense mechanisms against various pathogens. Antibodies of the IgG2 subclass are mainly produced against antigens of a polysaccharide nature, anti-Rhesus antibodies are IgG4.

IgG molecules freely diffuse from the blood plasma into the tissue fluid, where almost half (48.2%) of the IgG present in the body is located.

The rate of IgG biosynthesis is 32 mg / kg of body weight per day, the half-decay period is 21-23 days. The exception is IgG3, for which the half-decay period is much shorter - 7-9 days.

The transplacental transition of IgG is provided by a special grouping of the Fc fragment. Antibodies passing through the placenta from mother to child are essential for protecting the child's body from a number of microbes and toxins: pathogens of diphtheria, tetanus, poliomyelitis, measles. By the end of the first year of a child's life, the blood contains 50-60% of IgG of their content in an adult, by the end of the 2nd year - about 80% of that in adults.

Deficiency of IgG2 and IgG4 in the first years of life determines the high sensitivity of the child to the pathogenic effects of pneumococci, meningococci and other pathogens.

Immunoglobulins class A

In accordance with the structural features, three types of class A immunoglobulins are distinguished:

 serum IgA, having a monomeric structure and constituting 86% of all IgA contained in serum;

 serum dimeric IgA;

 secretory IgA, which is a polymer, most often a dimer, is characterized by the presence of an additional secretory component that is absent in serum IgA.

IgA is not detected in the secretions of newborns; in saliva, they appear in children at the age of 2 months. The content of secretory IgA in saliva reaches its level in an adult by the age of 8. By the end of the first year of a child's life, the blood contains approximately 30% IgA. Plasma IgA level reaches that in adults by 10-12 years. Class A immunoglobulins make up about 20% of the total amount of immunoglobulins.

Normally, the IgG / IgA ratio in blood serum is 5-6, and in secreted biological fluids (saliva, intestinal juice, milk) it decreases to 1 or less. IgA is contained in an amount of up to 30 mg per 100 ml of secretion.

By physical and chemical properties IgA are heterogeneous, can occur in the form of monomers, dimers and tetramers with sedimentation constants 7, 9, 11, 13. In blood serum, IgA are presented mainly in the monomeric form; serum IgA is synthesized in the spleen, lymph nodes and mucous membranes.

The biological function of IgA is mainly in the local protection of mucous membranes from infection. The antigens penetrated under the epithelium meet dimeric IgA molecules. The resulting complexes are actively carried to the surface of the mucous membranes after they are combined with the transport fragment in the epithelial membranes.

It has been suggested that it is possible to activate complement with the participation of IgA in an alternative way and, thus, ensure the processes of opsonization and lysis of bacteria with the participation of IgA.

It is also known that secretory IgA prevents bacteria from adhering to epithelial cells, thus making it difficult for bacteria to colonize mucous membranes.

In addition to secretory IgA, IgM and IgG contained in secretions in humans are essential, and IgM can be actively secreted due to the presence of a secretory component and play important role in providing local immunity in digestive tract... IgG can enter secretions only passively.

The system of secretory immunoglobulins provides an intense, but short-lived immune response and does not form immunological memory cells, prevents the contact of antigens with plasma IgG and IgM, subsequent activation of complement and cytolytic destruction of one's own tissues.

Class D immunoglobulins

Class D immunoglobulins make up about 2% of the total amount of immunoglobulins in the blood. Their concentration in serum reaches 30 mg / l, MM is, according to different authors, from 160 "000 to 180" 000; sedimentation constants range from 6.14 to 7.04 S. IgD does not bind complement, does not pass through the placenta and does not bind tissues. 75% of IgD is found in blood plasma, the half-life is 2.8 days, the biosynthesis rate is 0.4 mg / kg per day. The biological function of IgD is unclear; at certain stages of differentiation of B-lymphocytes, IgD play the role of a receptor. The IgD concentration almost doubles during pregnancy, and also increases in some chronic inflammatory processes.

Class E immunoglobulins

The concentration of IgE in plasma is 0.25 mg / l, the percentage of the total amount of immunoglobulins is 0.003%, the half-life is 2.3 - 2.5 days; biosynthesis rate - 0.02 mg / kg of mass per day.

IgE does not bind complement, does not pass through the placenta, is thermolabile, quickly and firmly binds to allogeneic tissues, does not precipitate antigens. In allergic diseases, the IgE concentration rises sharply and reaches an average of 1.6 mg / l.

Plasma cells that synthesize IgE are found mainly in the mucous membranes of the bronchi and bronchioles, gastrointestinal tract, Bladder, in the tonsils and adenoid tissue. The distribution of IgE-producing cells is similar to that of IgA-producing cells.

In the case of overcoming the barrier formed by secretory IgA, the antigen interacts with IgE - antibodies fixed on mast cells, the development of allergic reactions... The concentration of IgE in the blood reaches adult levels by about 10 years of age. With the participation of the Fc-fragment, IgE are fixed on the cell surface at the expense of Fc-receptors.

There are classical high-affinity receptors of mast cells and basophils for IgE, and from 30ґ103 to 400ґ103 IgE molecules, as well as low-affinity receptors, can be fixed on one basophil. The latter are represented mainly on macrophages, eosinophils, platelets.

Antibodies of the IgE class are responsible for the development of anaphylactic (atopic) allergic reactions of the humoral type.

It should be noted that only about 1% of IgE is present in the blood, more than 99% of IgE is secreted by enterocytes into the intestinal lumen, and IgE secreted into the intestinal lumen creates antihelminthic protection, in particular, due to IgE-dependent cytolysis provided by eosinophils. As you know, eosinophils can produce two toxic proteins - the large basic protein and the cationic protein of eosinophils.