Although antigens are large molecules, only certain regions of antigens, usually sections of the antigen 10 amino acids long, serve in stimulating lymphocytes to produce antibodies. These regions, called antigenic determinants, interact with receptor sites on the lymphocytes. Antigens may have one or more antigenic determinant regions and, thus, may stimulate one or more antibodies to be produced.
Only certain portions of antibodies act as the binding sites for antigens. There are two antigen binding sites on each antibody. All antibody molecules have four polypeptide chains, two longer, identical "heavy" chains and two shorter, identical "light" chains. The chains are made from globulin proteins, especially from gamma globulin proteins. The two heavy chains are linked together by disulfide bonds and paired up with the two light chains which are linked to the heavy chains by disulfide bonds as well. Antigens are bound to the antibody between the free ends of each pair of heavy and light chains.
The main portion of both pairs of chains (three quarters of length of the heavy chains and half of length of the light chains) have great constancy in their amino acid sequence. Only at the two antigen binding sites does variability in amino acid sequence occur. Variation in antibody specificity probably occurs due to a combination of factors encapsulated by two opposing theories: the Clonal Selection theory, which argues that a different gene is required for each different binding site needed to handle all the different possible antigens, and the somatic mutation theory which argues
