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BELARUSIAN STATE UNIVERSITY

Department of Biology

ANTIBODIES, CLASSIFICATION AND FUNCTIONS

abstract

student of the 4th year of the 6th group

KOVALCHUK K.V.

Minsk 2004

Discovery of antibodies

The structure of antibodies

Classification of antibodies

Functions of antibodies

Literature

Discovery of antibodies

The term "antibody" was coined in late XIX century. In 1890, Behring and Kitasato conducted experiments in which they studied the effects of diphtheria and tetanus toxins on guinea pigs. They injected animals with a sublethal dose of the toxin, after a while they took their serum and injected it along with the lethal dose of the toxin into other animals, as a result of which the animals did not die. It was concluded that after immunization with a toxin, a substance appears in the blood of animals that can neutralize it and thereby prevent the disease. This substance was called an antitoxin, and then a more general term was introduced - an antibody; Substances that produce antibodies are called antigens.

It was only in 1939 that Tiselius and Kabat showed that antibodies are contained in a specific fraction of serum proteins. They immunized the animal with ovalbumin and took two samples from the obtained serum, ovalbumin was added to one of them and the precipitate formed (antibody-ovalbumin complex) was removed. Electrophoresis revealed that in the sample where ovalbumin was added, the content of g-globulins was significantly lower than in the other sample. This indicated that the antibodies are g-globulins. To distinguish them from other proteins contained in this fraction of globulins, the antibodies were named immunoglobulins. It is now known that antibodies are also found in significant amounts in fractions of b- and b-globulins.

The structure of antibodies has been established in a variety of experiments. Basically, they consisted in the fact that the antibodies were processed with proteolytic enzymes (papain, pepsin), and subjected to alkylation and reduction with mercaptoethanol. Then the properties of the obtained fragments were studied: their molecular weight (chromatography), quaternary structure (X-ray diffraction analysis), ability to bind to an antigen, etc. were determined. Antibodies to these fragments were also used: it was found out whether antibodies to one type of fragments can bind to fragments of another type. Based on the obtained data, the model of the antibody molecule described below was constructed.

The structure of antibodies

An antibody molecule consists of four polypeptide chains (Fig. 1): two heavy (H; molecular weight 50-70 kDa) and two light (L; molecular weight 23 kDa). The chains are connected by non-covalent bonds (hydrophobic bonds) and disulfide bridges and consist of two (light chain) or four (heavy chain) domains about 110 amino acid residues long. The variable domains VH and VL, which are the N-terminal portions of the chains, form an antigen-binding site. In addition to them, light chains contain one (CL) each, and heavy chains each contain three or four (CH1-4) constant domains.

Enzymatic cleavage of antibodies with the proteolytic enzyme papain produces three fragments: two identical antigen-binding fragments (Fab) and one crystallizable fragment (Fc). The Fab fragment consists of an intact L chain disulfide-linked to the CH1 and VH domains, and its N-terminal portion (Fv fragment) has antigen-binding activity. The Fc fragment consists of two pairs of CH2 and CH3 domains connected by a disulfide bond. This fragment is not involved in the binding of antigens, but performs effector functions - reacting with cells and complement factors.

The ability of an antibody to bind to a particular antigen is determined by the amino acid composition of the variable domains, or rather their hypervariable regions. These sites are characterized by a very high variability in the sequence of amino acids. Each VH and VL domain contains three hypervariable regions, which actually form antigen-binding sites. The sequences between them are called frame sequences; they are characterized by lower structural variability.

Rice. 1. The structure of the antibody molecule. H and L, heavy and light chains; CDR, hypervariable regions.

The amino acid sequence of the constant region varies slightly. Light chain sequencing revealed the existence of two main variants of the amino acid sequences of the CL domains, which led to the isolation of two types of light chains - kappa (k) and lambda (l). An antibody molecule can simultaneously contain either two k-chains or two l-chains (k-chains are more common in human antibodies).

Also, the determination of amino acid sequences made it possible to distinguish five types of CH-regions and, accordingly, heavy chains (b, e, f, d, l). Chains m and e each contain four constant domains, the remaining chains contain three constant domains, as well as a hinge region between the CH1 and CH2 domains. Depending on what type of heavy chain the antibody contains, there are five classes of immunoglobulins: IgA (heavy chain type b), IgD (e), IgE (e), IgG (d), IgM (m). Due to some differences in amino acid sequences, several types of l-chains are distinguished, as well as several types of b- and g-chains (and, accordingly, several subclasses of IgG and IgA). Heavy chains (primarily CH2 domains) are associated with several oligosaccharide chains, which likely increase the solubility of antibodies and are involved in binding to complement components and cellular receptors.

In domains, polypeptide chains are stacked forming β-pleated layers, in which antiparallel chains are connected by loops (Fig. 2). These loops can have different lengths and amino acid sequences, which is very important because they form the antigen-binding site. Within each domain, two β-layers are linked by a disulfide bond and stabilized by hydrophobic interactions. The Y-shaped quaternary structure (Fig. 3) is formed due to non-covalent interactions between domains. Carbohydrate molecules are located between the CH2 domains, which makes these domains protrude and more accessible for interaction with a variety of molecules, such as components of the complement system.

Fig.2. Two-dimensional layout of the polypeptide chain within the VL domain: two p-pleated layers connected by a disulfide bond (black stripe).

Fig.3. Diagram showing the interaction between light and heavy chain domains. Carbohydrate molecules are located between the CH2 domains. Hypervariable regions (CDRs) are shown.

Classification of antibodies

As mentioned above, five classes of immunoglobulins are distinguished depending on the type of heavy chain.

IgG make up the majority of serum antibodies. Most of the antibodies of the secondary immune response and antitoxins are represented precisely by class G immunoglobulins. Maternal IgG provide passive immunity to the child in the first few months of life, entering the blood of the fetus through the placenta. IgGs activate the complement system and bind to cell surface antigens, thereby making these cells more accessible to phagocytosis (opsonization). Able to bind to tissues causing anaphylactic reactions.

IgM molecules consist of five identical four-stranded subunits connected by disulfide bonds. They also contain an additional polypeptide chain (J-chain), which forms an immunoglobulin-type domain and is linked by disulfide bonds to the C-terminal peptides (18 amino acid residues) of the heavy chains of individual monomers. Presumably, it is involved in the polymerization of monomers. Class M immunoglobulins are found predominantly in the blood. They dominate as “early” antibodies (the first to appear during the development of an immune response). Due to the many binding sites cause cell agglutination. More effective than IgG in activating complement.

IgA predominate among the antibodies of serous-mucous secretions (saliva, colostrum, milk, respiratory secretions), where they are mainly represented by a dimeric form. Like IgM, they contain a C-terminal peptide, to which a J-chain can join, linking two monomers into a dimer. This complex additionally binds a protein called the secretory component, which facilitates the delivery of antibodies to the secrets and protects them from proteolysis. In human serum, they are represented mainly by the monomeric form, and in the serum of other mammals, mainly by the dimer. Prevent the penetration of viruses, microorganisms through the mucous membranes.

IgD And IgE present in serum at very low concentrations. IgDs are often found on the cytoplasmic membranes of B cells and are thought to be involved in antigen-dependent lymphocyte differentiation. IgE are found on the membranes of basophils and mast cells. They participate in allergic reactions, causing the secretion of histamine and other vasoactive substances by the IgE carrier cell in response to the binding of the IgE molecule to the antigen. Possibly play a significant role in anthelminthic immunity.

Functions of antibodies

Antibodies are synthesized by B-lymphocytes and plasma cells formed from them. Their molecules are embedded in the cytoplasmic membrane of B-lymphocytes, where they function as antigen-specific receptors. Most human B-lymphocytes express on their surface immunoglobulins of two classes - IgM and IgD. But in certain areas of the body, B cells carrying antibodies of other classes (for example, IgA in the intestinal mucosa) can occur with a high frequency. Plasma cells secrete antibodies into blood plasma and tissue fluid. All antibodies produced by a single B cell (or plasma cell) have an identical antigen-binding site and can only bind to one antigen.

The primary function of antibodies is to bind to foreign (normally) antigens with their subsequent inactivation. Antibodies are able to inactivate toxins by binding to areas of the toxin molecule responsible either for adsorption on cell receptors or directly for toxic effects. Similarly, the binding of antibodies to proteins necessary for adsorption of the virus to cell receptors leads to the inactivation of virions.

In addition, antibodies are able to involve other elements of the immune system in the immune response: the complement system and host cells. Complement component C1q is able to bind to the constant domains of the heavy chain of antibodies of classes G and M (with the CH2 and CH3 domains, respectively). This causes a cascade of reactions (the process of complement activation along the classical pathway), ultimately leading to the lysis of the cell whose antigens were bound by the antibodies. Some body cells carry Fc receptors on their surface, to which antibody molecules can bind via the Fc fragment. Macrophages have these receptors, which allows them to recognize antigen-antibody complexes with their subsequent phagocytosis (antibodies are opsonins, i.e. molecules that, when bound to antigens, facilitate their phagocytosis). Also, the Fc fragment is responsible for the fixation of antibodies on the cells of certain tissues and the development of anaphyloxic reactions.

To any antigens in the body of the animal initially there are antibodies. This suggests that each organism produces millions of different immunoglobulins, differing in their antigen binding sites. This diversity is provided by several mechanisms. Light and heavy chains of antibody molecules are encoded by several types of gene segments: light chain - by three types of segments (V, J, C), heavy - by four (V, D, J, C). The genome usually contains from several to several hundred segments of each type, slightly differing in nucleotide sequence. To synthesize an entire polypeptide (light or heavy chain), it is necessary to combine the nucleotide sequences of segments of each type. Such association occurs first at the DNA level (somatic recombination) and then at the messenger RNA level (splicing). As a result, a huge number of mRNA variants and, accordingly, polypeptide chains are formed. During somatic recombination and splicing, insertions and deletions of nucleotides can occur, which, together with an increased frequency of mutations in antibody genes, further increases the diversity of these unique proteins.

Literature

1. Immunology / Roit A., Brostoff J., Mail D.-M.: Mir, 2000.-592 p.

2. Immunology: In 3 volumes; v.1 / Ed. W. Paula.-M.: Mir, 1987-88.-476 p.

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ANTITOXINS(Greek anti- against + toxins) - specific antibodies formed in the body of humans and animals under the influence of toxins (anatoxins) of microbes, poisons of plants and animals, which have the ability to neutralize their toxic properties.

Antitoxins are one of the immunity factors (see) and play a major protective role in toxin infections (tetanus, diphtheria, botulism, gas gangrene, some streptococcal and staphylococcal diseases, etc.).

In 1890, Behring and Kitasato (E. Behring, S. Kitasato) observed for the first time that the serum of animals repeatedly treated with non-lethal doses of diphtheria and tetanus toxin acquired the ability to neutralize these toxins (see). In the Pasteur Institute in Paris, Roux (E. Roux) in 1894 received the first antitoxic diphtheria serum, which he was the first to introduce into widespread practice. Antitoxic serum against gas gangrene was obtained by Weinberg (M. Weinberg in 1915 by immunizing animals with increasing doses of live culture. After the discovery by G. Ramon in 1923 of toxoids (see), obtaining any antitoxins does not encounter great difficulties.

In the body, under natural conditions, antitoxins are formed as a result of a toxinemic infection or as a result of the carriage of toxigenic microorganisms, they are found in the blood serum and can provide immunity to toxinemic infections.

Antitoxic immunity can also be created artificially: active immunization with toxoid or the introduction of antitoxic serum (passive immunity). During primary immunization with toxoid, the rate of formation of antitoxins depends on the sensitivity of the immunized, on the dose and quality of the toxoid, on the intervals and rate of resorption of the antigen in the body. When immunized with sorbed or precipitated toxoids used in the present, the appearance and accumulation of antitoxins in the blood occurs more slowly than when immunized with the same doses of unsorbed toxoids, but the titers of antitoxins are much higher and are detected for a longer time. After primary immunization, the "immunological memory" in the body for the formation of antitoxins is preserved indefinitely, up to 25 years, and possibly for a lifetime. With revaccination, the production of antitoxins in the body occurs very quickly. Already on the 2nd day after revaccination, significant amounts of antitoxins are detected, the titers of which continue to increase in the next 10-12 days. The rapid production of antitoxins during revaccination has a large practical value in the prevention of tetanus and other toxin infections. In order to prevent tetanus in newborns, pregnant women are immunized and revaccinated with tetanus toxoid. The resulting antitoxins have the ability to pass through the placenta into the body of the fetus, and also be transmitted to the newborn with mother's milk.

Antitoxic sera are obtained by immunizing horses and cattle with increasing doses of toxoids, and then with the corresponding toxins. The formation of antitoxins in animals occurs more intensively in the case of the use of precipitated antigens - 1% calcium chloride or 0.5% potassium-aluminum alum. To increase the titer of antitoxins in producing horses, various stimulants are used (see Adjuvants).

Soviet scientists (O. A. Komkova, K. I. Matveev, 1943, 1959) developed a method for obtaining polyvalent anti-gangrenous (Cl. perfringens, Cl. oedematiens, Cl. septicum) and anti-botulinum antitoxins of types A, B, C and E from one producer. In this case, the horse is immunized with small doses of several antigens. This method has found wide application in the practice of producing polyvalent antigangrenous and antibotulinum sera from one producer with satisfactory titers of all antitoxins.

Antitoxins of antidiphtheria and antitetanus horse serum are mainly contained in γ1-, γ2-, β2-fractions of globulins.

Antitoxins in practical medicine are used for the prevention and treatment of diphtheria, tetanus and botulism. With the help of antitoxins in humans, it is possible to create passive immunity of such tension that it protects against disease in the event that an infectious agent or toxin enters the body, as happens with botulism. Children who have been in contact with someone with diphtheria are given antitoxins to prevent diphtheria. Anti-tetanus serum is administered to non-immunized children and adults in trauma. When cases of botulism are detected, all persons who have eaten the product that caused the disease are administered polyvalent anti-botulinum serum for prevention.

To obtain a therapeutic effect, it is very important to early administration of an antitoxin capable of neutralizing the toxin circulating in the blood. Therefore, the effectiveness of serotherapy (see) depends largely on the period of use of antitoxins. The results of treatment with antitoxins in different infections are not the same. In the treatment of diphtheria in humans, good results have been obtained; in the treatment of tetanus and botulism, the best results are obtained with the introduction of antitoxins at the onset of the disease. Treatment of staphylococcal sepsis with homologous alpha-staphylococcal antitoxin is effective (S. V. Skurkovich, 1969). In gas gangrene, the therapeutic effect of antitoxins is questioned, although many doctors continue to use it.

However, administration of heterologous antitoxic sera to humans for the prevention and treatment of infections is sometimes accompanied by complications. In rare cases, with the introduction of horse serum, a person may develop anaphylactic shock (see), sometimes fatal. Serum sickness develops in 5-10% of cases (see). Therefore, in the USSR and other countries, for the prevention of tetanus in humans, instead of horse serum, homologous immunoglobulin from donor blood containing tetanus antitoxin is used. Homologous antitoxin rarely causes adverse reactions and is in the body in the required titer up to 30-40 days (K. I. Matveev, S. V. Skurkovich et al., 1973).

To eliminate the complications observed from the introduction of heterologous native antitoxic sera, various methods have been proposed for cleaning A. from ballast proteins: salting out with neutral salts, fractionation using electrodialysis, and digestion with enzymes. top scores were obtained by peptic digestion (I. A. Perfentiev, 1936). Purification of antitoxic sera by proteolysis in the USSR was carried out at the Institute of Epidemiology and Microbiology. Η. F. Gamalei of the Academy of Medical Sciences of the USSR (A. V. Beilinson and colleagues, 1945). The advantage of the proteolysis method (diaferm-3) is that it gives a 2-4 times greater degree of purification of antitoxins than other methods, but 30-50% of antitoxins are lost. During proteolysis, a profound change in the antitoxin molecule and a decrease in its anaphylactogenic properties occur. Methods have been developed for the purification and concentration of antitoxins using aluminum hydroxide, filtration through Sephadex (molecular sieves) and the use of ion exchange. At t° 37° for 20 days, the antitoxin titer in purified sera decreases slightly, then stabilizes and remains unchanged for up to 2 years or more. After freeze-drying under vacuum at low temperatures, the antitoxin titer is reduced by 2-25%. Dried antitoxins retain their physical and specific properties and can be stored for a number of years.

Antitoxins are subject to mandatory safety control in guinea pigs and non-pyrogenicity in rabbits.

The content of antitoxins in antitoxic sera is expressed in international units (IU) adopted by the World Health Organization, which corresponds to the minimum amount of serum that neutralizes the standard unit of the toxin, expressed in minimum lethal, necrotic or reactive doses, depending on the type of animal and toxin. For example, the ME of tetanus toxoid corresponds to the minimum amount that neutralizes approximately 1000 minimum lethal doses (Dim) of a standard 350 g guinea pig toxin; ME of antibotulinum antitoxin - the smallest amount of serum that neutralizes 10,000 Dim of the toxin for mice weighing 18-20 g; The ME of the standard anti-diphtheria serum corresponds to its minimum amount that neutralizes 100 Dim of the standard toxin for a 250 g guinea pig.

For some serums that do not have accepted international standards, national standards have been approved, and their activity is expressed in national units, which are called antitoxic units (AU).

When titrating antitoxins, first determine the conditional (experimental) unit of the toxin. The experimental dose of the toxin is denoted by the symbol Lt (Limes tod) and is set in relation to the standard antitoxic serum produced by the State. Scientific Research Institute for Standardization and Control of Medical Biological Preparations. L. A. Tarasevich M3 USSR. To determine the experimental dose of the toxin, decreasing or increasing doses of the toxin in a volume of 0.3 ml are added to a certain amount of standard serum in accordance with the level of titration (to 1/5, 1/10 or 1/50 ME) in a volume of 0.2 ml. After keeping at room temperature for 45 minutes, this mixture is administered intravenously to white mice in a volume of 0.5 ml per mouse. Animals are observed for 4 days. The experimental dose is taken to be the minimum amount of toxin that, when mixed with the accepted dose of standard serum, causes the death of 50% of the mice taken in the experiment.

Anti-botulinum antitoxic sera of types A, B, C, E and anti-gangrenous (Cl. perfringens) B, C are titrated at the level of 1/5 IU. The experimental dose of the toxin is also titrated to 1/5 IU of the standard serum. Anti-botulinum serum type F and anti-gangrenous sera types A, D, E, as well as anti-tetanus serum are titrated at the level of 1/10 IU. The experimental dose of the toxin is necessarily titrated to 1/10 IU of standard serum. Antigangrenous serum (Cl. oedematiens) is titrated at the level of 1/50 IU. The experimental dose of the toxin is titrated to 1/50 IU of standard serum. The test sera are diluted depending on the expected titer and an experimental dose of toxin in a volume of 0.3 ml (per 1 mouse) is added to various dilutions of serum in a volume of 0.2 ml, the mixture is left to combine at room temperature for 45 minutes. and administered 0.5 ml intravenously to white mice. Anti-tetanus serum is titrated by subcutaneous injection of 0.4 ml of the mixture into the hind paw of the mouse. At least two mice are taken into the experiment for each dose, the mixture is prepared at the rate of at least 3 mice. With each serum titration, the activity control of the experimental dose of the toxin with standard serum is mandatory.

The principles of titration of diphtheria antitoxin are the same as for other sera, only dilutions of standard serum and an experimental dose of toxin are jointly administered intradermally to a guinea pig (Roemer's method). The so-called necrotic dose - limes necrosis (Ln) of diphtheria toxin is preliminarily titrated with standard serum, which is the smallest amount of toxin that, when administered intradermally to a guinea pig (in a volume of 0.05 ml) in a mixture with 1/50 IU of standard anti-diphtheria serum, causes by the 4-5th day, the formation of necrosis. Titration of diphtheria antitoxin according to the Ramon method (flocculation reaction) is carried out using a toxin or anatoxin, in which the content of antigenic units (AU) in 1 ml is preliminarily determined. One antigenic unit of the toxin, designated as the flocculation threshold - limes flocculationis (Lf), is neutralized by one unit of diphtheria antitoxin. For the titration of small amounts of diphtheria antitoxin, the intradermal Jensen method on rabbits is also used.

Antitoxins are widely used for the prevention and treatment of toxin infections. In addition, they are used to neutralize the poisons of snakes, spiders and poisons of plant origin.

Bibliography: Ramon G. Forty years research work, per. from French, Moscow, 1962; Rezepov F. F. and others. Determination of the safety and specific activity of immune sera and globulins, in the book: Methodical. laboratory manual. bact quality assessment. and viral. drugs, ed. S. G. Dzagurova, p. 235, M., 1972; Toxins-anatoxins and antitoxic serums. M., 1969; Behring and. To i t a in a t o, Über das Zustandekommen der Diphterie-Immunität und der Tetanus-Immunität bei Tieren, Dtsch. med. Wschr., S. 1113, 1890; Kuhns W. J. a. Pappenheimer A. M. Immunochemical studies of antitoxin produced in normal and allergic individuals hyperimmunized with diphtheria toxoid, J. exp. Med., v. 95, p. 375, 1952; Miller J.F.A.P.a. o. Interaction between lymphocytes in immune responses, Cell. Immunol., v. 2, p. 469, 1971, bibliogr.; White R. G. The relation of the cellular responses in germinal or lymphocytopoietic centers of lymph nodes to the production of antibody, in: Mechanism. antibody formation, p. 25, Prague, 1960.

K. I. MATVEEV

in response to the presence of antigens. For each antigen, specialized plasma cells corresponding to it are formed, which produce antibodies specific for this antigen. Antibodies recognize antigens by binding to a specific epitope - a characteristic fragment of the surface or linear amino acid chain of the antigen.

Antibodies are composed of two light chains and two heavy chains. In mammals, five classes of antibodies (immunoglobulins) are distinguished - IgG, IgA, IgM, IgD, IgE, differing from each other in the structure and amino acid composition of heavy chains and in effector functions performed.

History of study

The very first antibody was discovered by Bering and Kitazato in 1890, however, at that time, nothing definite could be said about the nature of the discovered tetanus antitoxin, except for its specificity and its presence in the serum of an immune animal. Only from 1937 - the studies of Tiselius and Kabat, did the study of the molecular nature of antibodies begin. The authors used the method of protein electrophoresis and demonstrated an increase in the gamma globulin fraction of the blood serum of immunized animals. Adsorption of serum by antigen, which was taken for immunization, reduced the amount of protein in this fraction to the level of intact animals.

The structure of antibodies

Antibodies are relatively large (~150 kDa - IgG) glycoproteins with a complex structure. Consist of two identical heavy chains (H-chains, in turn, consisting of V H, C H1, hinge, C H2 and C H3 domains) and two identical light chains (L-chains, consisting of V L and C L domains). Oligosaccharides are covalently attached to the heavy chains. Antibodies can be cleaved into two Fabs using papain protease. fragment antigen binding- antigen-binding fragment) and one (eng. crystallizable fragment- a fragment capable of crystallization). Depending on the class and functions performed, antibodies can exist both in monomeric form (IgG, IgD, IgE, serum IgA) and in oligomeric form (dimer-secretory IgA, pentamer - IgM). In total, there are five types of heavy chains (α-, γ-, δ-, ε- and μ-chains) and two types of light chains (κ-chain and λ-chain).

Heavy chain classification

There are five classes ( isotypes) immunoglobulins that differ:

• magnitude
• charge
• amino acid sequence
• carbohydrate content

The IgG class is classified into four subclasses (IgG1, IgG2, IgG3, IgG4), the IgA class into two subclasses (IgA1, IgA2). All classes and subclasses make up nine isotypes that are normally present in all individuals. Each isotype is defined by the amino acid sequence of the heavy chain constant region.

Functions of antibodies

Immunoglobulins of all isotypes are bifunctional. This means that any type of immunoglobulin

• recognizes and binds antigen, and then
• enhances the killing and/or removal of immune complexes formed as a result of activation of effector mechanisms.

One area of ​​the antibody molecule (Fab) determines its antigenic specificity, and the other (Fc) performs effector functions: binding to receptors that are expressed on body cells (for example, phagocytes); binding to the first component (C1q) of the complement system to initiate the classical pathway of the complement cascade.

This means that each lymphocyte synthesizes antibodies of only one specific specificity. And these antibodies are located on the surface of this lymphocyte as receptors.

As experiments show, all cell surface immunoglobulins have the same idiotype: when a soluble antigen, similar to polymerized flagellin, binds to a specific cell, then all cell surface immunoglobulins bind to this antigen and they have the same specificity, that is, the same idiotype.

The antigen binds to receptors, then selectively activates the cell with the formation of a large number of antibodies. And since the cell synthesizes antibodies of only one specificity, this specificity must coincide with the specificity of the initial surface receptor.

The specificity of the interaction of antibodies with antigens is not absolute, they can cross-react with other antigens to varying degrees. Antiserum obtained against one antigen may react with a related antigen carrying one or more of the same or similar determinants. Therefore, each antibody can react not only with the antigen that caused its formation, but also with other, sometimes completely unrelated molecules. The specificity of antibodies is determined by the amino acid sequence of their variable regions.

Clonal selection theory:

1. Antibodies and lymphocytes with the desired specificity already exist in the body before the first contact with the antigen.
2. Lymphocytes that participate in the immune response have antigen-specific receptors on the surface of their membrane. B-lymphocytes have receptors, molecules of the same specificity as the antibodies that lymphocytes subsequently produce and secrete.
3. Any lymphocyte carries on its surface receptors of only one specificity.
4. Lymphocytes that have antigen go through the stage of proliferation and form big clone plasma cells. Plasma cells synthesize antibodies only of the specificity for which the progenitor lymphocyte has been programmed. Proliferation signals are cytokines, which are secreted by other cells. Lymphocytes can secrete cytokines themselves.

Antibody variability

Antibodies are extremely variable (up to 10 8 variants of antibodies can exist in the body of one person). All the diversity of antibodies results from the variability of both heavy chains and light chains. Antibodies produced by one or another organism in response to certain antigens are distinguished:

• isotypic variability - manifested in the presence of classes of antibodies (isotypes) that differ in the structure of heavy chains and oligomerism, produced by all organisms of a given species;
• Allotypic variability - manifested at the individual level within a given species in the form of variability of immunoglobulin alleles - is a genetically determined difference of a given organism from another;
• idiotic variability - manifested in the difference in the amino acid composition of the antigen-binding site. This applies to the variable and hypervariable domains of the heavy and light chains that are in direct contact with the antigen.

Proliferation control

The most effective control mechanism is that the reaction product simultaneously serves as its inhibitor. This type of negative feedback occurs in the formation of antibodies. The action of antibodies cannot be explained simply by neutralization of the antigen, because whole IgG molecules suppress antibody synthesis much more efficiently than F (ab ") 2 fragments. It is assumed that the blockade of the productive phase of the T-dependent B-cell response occurs as a result of the formation of cross-links between the antigen , IgG and Fc - receptors on the surface of B-cells.Injection of IgM enhances the immune response.Since antibodies of this particular isotype appear first after the introduction of the antigen, then on early stage immune response, they are credited with a reinforcing role.

• A. Roit, J. Brusstoff, D. Meil. Immunology - M.: Mir, 2000 - ISBN 5-03-003362-9
• Immunology in 3 volumes / Pod. ed. W. Paul.- M.: Mir, 1988
• V. G. Galaktionov. Immunology - M.: Ed. Moscow State University, 1998 - ISBN 5-211-03717-0

see also

• Abzymes are catalytically active antibodies.
• Avidity, affinity - antigen and antibody binding characteristics

Antibodies(immunoglobulins, IG, Ig) are soluble glycoproteins present in blood serum, tissue fluid or on the cell membrane that recognize and bind antigens. Immunoglobulins are synthesized by B-lymphocytes (plasma cells) in response to foreign substances of a certain structure - antigens. Antibodies are used by the immune system to identify and neutralize foreign objects such as bacteria and viruses.

Antibodies perform two functions: an antigen-binding function and an effector function (for example, launching classical scheme complement activation and cell binding), are the most important factor in specific humoral immunity, consist of two light chains and two heavy chains. In mammals, there are five classes of immunoglobulins - IgG, IgA, IgM, IgD, IgE, which differ in the structure and amino acid composition of heavy chains. Immunoglobulins are expressed as membrane-bound receptors on the surface of B cells and as soluble molecules present in serum and tissue fluid.

The structure of antibodies

Antibodies are relatively large (~150 kDa - IgG) glycoproteins with a complex structure. Consist of two identical heavy chains (H chains, in turn consisting of VH, CH1, hinge, CH2 and CH3 domains) and two identical light chains (L chains, consisting of VL and CL domains). Oligosaccharides are covalently attached to the heavy chains. With the help of papain protease, antibodies can be split into two Fab (eng. fragment antigen binding - antigen-binding fragment) and one Fc (eng. fragment crystallizable - a fragment capable of crystallization). Depending on the class and functions performed, antibodies can exist both in monomeric form (IgG, IgD, IgE, serum IgA) and in oligomeric form (dimer-secretory IgA, pentamer - IgM). In total, there are five types of heavy chains (α-, γ-, δ-, ε- and μ-chains) and two types of light chains (κ-chain and λ-chain).

Types of antibodies:

• IgG is the main immunoglobulin in the serum of a healthy person (accounts for 70-75% of the entire fraction of immunoglobulins), is most active in the secondary immune response and antitoxic immunity. Due to its small size (sedimentation coefficient 7S, molecular weight 146 kDa), it is the only immunoglobulin fraction capable of transporting through the placental barrier and thus providing immunity to the fetus and newborn.
• IgM are a pentamer of the basic four-strand unit containing two μ-strands. Appear during the primary immune response to an unknown antigen, up to 10% of the immunoglobulin fraction. They are the largest immunoglobulins (970 kDa).
• IgA Serum IgA makes up 15-20% of the total immunoglobulin fraction, while 80% of IgA molecules are present in monomeric form in humans. Secretory IgA is presented in a dimeric form in a complex with a secretory component, it is contained in serous-mucous secrets (for example, in saliva, colostrum, milk, secretions of the mucous membrane of the genitourinary and respiratory system).
• IgD makes up less than one percent of the plasma immunoglobulin fraction, is found mainly on the membrane of some B-lymphocytes. The functions are not fully understood, it is presumably an antigen receptor for B-lymphocytes that have not yet presented an antigen.
• IgE associated with the membranes of basophils and mast cells, in the free form in the plasma is almost absent. Associated with allergic reactions.

Functions of antibodies

Immunoglobulins of all isotypes are bifunctional. This means that any type of immunoglobulin - recognizes and binds the antigen, and then - enhances the killing and / or removal of immune complexes formed as a result of the activation of effector mechanisms. One area of ​​the antibody molecule (Fab) determines its antigenic specificity, and the other (Fc) performs effector functions: binding to receptors that are expressed on body cells (for example, phagocytes); binding to the first component (C1q) of the complement system to initiate the classical pathway of the complement cascade.

How antibodies are produced

The production of antibodies in response to the entry of antigens into the body depends on whether the body encounters this antigen for the first time or repeatedly. At the initial meeting, antibodies do not appear immediately, but after a few days, while IgM antibodies are first formed, and then IgG antibodies begin to predominate. The amount of antibodies in the blood reaches its peak in about a week, then their number slowly decreases. When the antigen enters the body again, the production of antibodies occurs faster and in a larger volume, while IgG antibodies are formed immediately. The immune system is able to remember its encounters with certain antigens for a very long time, which explains, for example, lifelong immunity to smallpox or childhood infections.

Antigen-antibody reaction

As a result of the antigen-antibody reaction in the gel, precipitation lines are formed, which can be used to judge the number of reacting components, the immunological relationship of antigens and their electrophoretic mobility. Antibodies can be detected in a macroscopic agglutination reaction using antigen-loaded particles. Numerous variants of immunological assays based on the interaction of labeled antigens and antibodies have been developed. Used as labels radioactive isotopes and enzymes.

How do antibodies neutralize toxins?

An antibody molecule, attached near the active center of a toxin, can stereochemically block its interaction with a substrate, especially with a macromolecular one. In complex with antibodies, the toxin loses its ability to diffuse in tissues and can become an object of phagocytosis, especially if the size of the complex increases as a result of binding to normal autoantibodies.

Protective effect of serum antibodies

Antibodies neutralize viruses in various ways - for example, stereochemically inhibiting the binding of the virus to the cellular receptor and thereby preventing its entry into the cell and subsequent replication. An illustration of this mechanism is the protective effect that antibodies specific to influenza virus hemagglutinin have. Antibodies to the hemagglutinin of the measles virus also prevent its penetration into the cell, but the intercellular spread of the virus is blocked by antibodies to the fusion protein of the cytoplasmic membranes of neighboring cells.

Antibodies can directly destroy viral particles by activating complement in the classical way or causing viral aggregation followed by phagocytosis and intracellular death. Even relatively low concentrations of antibodies in the blood can be effective: for example, it is possible to protect recipients from infection with polio by administering antiviral antibodies, or to prevent measles in contact children by administering normal human gamma globulin prophylactically.

maternal antibodies

In the first few months of life, when the child's own lymphoid system is still underdeveloped, protection against infections is provided by maternal antibodies that cross the placenta or come with colostrum and are absorbed in the intestines. The main class of milk immunoglobulins is secretory immunoglobulin A. It is not absorbed in the intestine, but remains here, protecting the mucous membrane. Strikingly, these antibodies are directed to bacterial and viral antigens that often enter the intestines. In addition, it is believed that cells producing immunoglobulin A to such antigens migrate to the breast tissue, from where the antibodies they produce enter the milk.

Antibodies: These are proteins produced by cells of the lymphoid organs (B lymphocytes) under the influence of an antigen and capable of entering into a specific relationship with them. In this case, antibodies can neutralize the toxins of bacteria and viruses, they are called antitoxins and virus-neutralizing antibodies.

They can precipitate soluble antigens - precipitins, stick together corpuscular antigens - agglutinins.

Nature of Antibodies: Antibodies are gamma globulins. In the body, gamma globulins are produced by plasma cells and make up 30% of all proteins in the blood serum.

Gammaglobulins that carry the function of antibodies are called immunoglobulins and are designated Ig. Ig proteins chemical composition belong to glycoproteins, that is, they consist of proteins, sugars, 17 amino acids.

Ig molecule:

Under electron microscopy, the Ig molecule has the shape of a Y with a changing angle.

The structural unit of Ig is a monomer.

The monomer consists of 4 polypeptide chains linked to each other by disulfide bonds. Of the 4 chains, two chains are long and curved in the middle. A molecular weight of 50-70 kD is the so-called heavy H chains, and two short chains are adjacent to the upper segments of the H chains, a molecular weight of 24 kD is light L chains.

Variable light and heavy chains together form a site that specifically binds to an antigen - an antigen-binding center Fab fragment, an Fc fragment responsible for complement activation.

Fab (eng. fragment antigen binding - antigen-binding fragment) and one Fc (eng. fragment crystallizable - a fragment capable of crystallization).

Classes of immunoglobulins:

Ig M - is from 5-10% of serum immunoglobulins. It is the largest molecule of all five classes of immunoglobulins. Molecular weight is 900 thousand kD. The first appears in the blood serum during the introduction of the antigen. The presence of Ig M indicates an acute process. Ig M agglutinates and lyses the antigen, and also activates complement. Tied to the bloodstream.

Ig G - is from 70-80% of serum immunoglobulins. Molecular weight 160 thousand kD. It is synthesized during a secondary immune response, is able to overcome the placental barrier and provide immune protection for newborns for the first time for 3-4 months, then it is destroyed. At the beginning of the disease, the amount of Ig G is insignificant, but as the disease progresses, their number increases. He owns the main role in protection against infections. High titers of Ig G indicate that the body is at the stage of recovery or a recent infection. It is found in blood serum and spreads through the intestinal mucosa in tissue fluid.

Ig A - ranges from 10-15%, molecular weight 160 thousand kD. plays important role in the protection of the mucous membranes of the respiratory and digestive tracts, the genitourinary system. There are serum and secretory Ig A. Serum neutralizes microorganisms and their toxins, does not bind complement and does not pass through the placental barrier.

Secretory Ig A activate complement and stimulate phagocytic activity in the mucous membranes, it is found mainly in the secretions of the mucous membranes, saliva, lacrimal fluid, sweat, nasal discharge, where it protects surfaces that communicate with the external environment from microorganisms. Synthesized by plasma cells. In human serum, it is represented by a monomeric form. Provides local immunity.

Ig E - its amount in serum is small and only a small part of plasma cells synthesize Ig E. They are formed in response to allergens and, interacting with them, cause a HNT reaction. Synthesized by B-lymphocytes and plasma cells. It does not pass through the placental barrier.

Ig D -participation is not well understood. Almost all is located on the surface of lymphocytes. Produced by cells of the tonsils and adenoids. IgD does not bind complement, does not pass through the placental barrier. Ig D and Ig A are interconnected and carry out the activation of lymphocytes. The concentration of Ig D increases during pregnancy, with bronchial asthma, with systemic lupus erythematosus.

Normal antibodies (natural)

The body contains a certain level of them, they are formed without the phenomena of antigenic stimulation. These include antibodies against erythrocyte antigens, blood groups, and against intestinal groups of bacteria.

The process of production of antibodies, their accumulation and disappearance have certain characteristics that are different in the primary immune response (this is the response upon initial encounter with the antigen) and the secondary immune response (this is the response upon repeated contact with the same antigen after 2-4 weeks).

The synthesis of antibodies in any immune response proceeds in several stages - these are the latent stage, logarithmic, stationary and the phase of reducing antibodies.

Primary immune response:

Latent phase: during this period, the process of recognition of the antigen and the formation of cells that are able to synthesize antibodies to it takes place. The duration of this period is 3-5 days.

Logarithmic phase: The rate of antibody synthesis is low. (duration 15-20 days).

Stationary phase: the titers of synthesized antibodies reach maximum values. Antibodies related to class M immunoglobulins are synthesized first, then G. Later, Ig A and Ig E may appear.

Decline phase: The level of antibodies decreases. Duration from 1-6 months.

Secondary immune response.