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A reaction that occurs when an antigen combines with a corresponding antibody to produce an immune complex. A substance that induces the immune system to form a corresponding antibody is called an immunogen. All immunogens are also antigens because they react with corresponding antibodies; however, an antigen may not be able to induce the formation of an antibody and therefore may not be an immunogen. For instance, lipids and all low-molecular-weight substances are not immunogenic. However, many such substances, termed haptens, can be attached to immunogens, called carriers, and the complex then acts as a new immunogen. See Antibody, Antigen
A molecule of antibody has two identical binding sites for one antigen or more, depending on its class. Each site is quite small and can bind only a comparably small portion of the surface of the antigen, which is termed an epitope. The specificity of an antibody for an antigen depends entirely upon the possession of the appropriate epitope by an antigen. The binding site on the antibody and the epitope on the antigen are complementary regions on the surface of the respective molecules which interlock in the antigen-antibody reaction. The intensity with which an antibody binds to the antigen depends on the exactitude of the fit between the respective binding site and epitope, as well as some inherent characteristics of the reacting molecules and factors in the environment. The epitope must be continuous spatially, but not structurally: in other words, if the molecule of the antigen consists of several chains, then an epitope may be formed by adjacent regions on two different chains, as well as by adjacent regions on the same chain. If the epitope is now modified either chemically (for example, by altering the hapten) or physically (for example, by causing the chains to separate), then its fit in the binding site will be altered or abolished, and the antigen will react with the antibody either less strongly or not at all.
The immune complex formed in the reaction consists of closely apposed, but still discrete, molecules of antigen and antibody. Therefore, the immune complex can dissociate into the original molecules. The proportion of the dissociated, individual molecules of antigen and antibody to those of the immune complex clearly depends on the intensity of the binding. These proportions can be measured in a standardized procedure, so that the concentration of antigen [Ag], antibody [Ab], and the immune complex [AgAb] becomes known. A fraction is then calculated and called either the dissociation constant or the association constant. The magnitude of either of these constants can be used subsequently to assess the intensity of the antigen-antibody reaction. See Immunoassay
Only one epitope of its kind generally occurs on each molecule of antigen, other than that which consists of multiple, identical units, though many epitopes of different configuration are possible. Particles, however, either natural ones such as cells or suitably treated artificial ones made of, for example, latex or glass, typically carry multiple identical epitopes, as well as nonidentical ones, because their surfaces contain many molecules of the same antigen. Immune complexes comprising many molecules eventually reach sufficient size to scatter light, at which point they can be detected by nephelometry or turbidimetry; if their growth continues, they become visible as precipitates, which can also be assayed by such methods as immunodiffusion. Since particles typically carry many molecules of antigen, they can be, in principle, aggregated and the reaction can be detected by inspection. Antigen-antibody reactions can also be detected at very low concentration of reactants through special techniques such as immunofluorescence and radioimmunoassay. See Immunoassay, Radioimmunoassay
The reaction between antigen and antibody is followed by a structural change in the remainder of the antibody molecule. The change results in the appearance of previously hidden regions of the molecule. Some of these hidden regions have specific functions, such as binding complement. Fixation of complement by immune complexes has been used to detect and measure antigen-antibody reactions. See Complement
The chief use of antigen-antibody reactions has been in the determination of blood groups for transfusion, serological ascertainment of exposure to infectious agents, and development of immunoassays for the quantification of various substances. See Blood groups, Immunology, Serology