Antibodies are globulin proteins produced by vertebrates in response to specific antigenic (immunogenic) challenge, and are a major feature of vertebrate immune defense against microbial disease. Antigens (immunogens) are substances capable of eliciting such responses because of their innate foreigness to the exposed animal. The nature of this foreigness relates to the presence of discrete chemical groupings (antigenic determinants or epitopes) on the surface of the antigenic molecule. All antigenic molecules consist of a mosaic of similar or dissimilar epitopes, with the number of epitopes representing the valency of the antigen. An important attribute of the antibody molecules is that they can react specifically with an antigen because they possess binding sites that are complementary to the antigenic epitopes. A good analogy is used by Feinberg (1 ), who refers to the epitope “bump” fitting the antibody “hollow.” Whereas the antigen is almost certainly multivalent, the majority of globulin antibody proteins in serum are divalent (IgG), i.e., possess two linked identical binding sites.
