Antibodies have a perfect fit with their corresponding antigens and are able to perform certain functions as a result of this binding. The antigen specificity of the antibody is determined by the variable (V) sequences of the 100 or so amino acid residues at the amino-terminal end of each of the heavy (H) and light (L) polypeptide chains of the antibody molecule (Fig 1); the remaining constant amino acid sequences from the constant (C) domains determine the class of antibodies to which the molecule belongs. Heterogeneity in the C domain distinguishes about 20 classes of immunoglobulin molecules, ie, isotypes, and distinguishes within each isotype a limited number of differences, ie, allotypes, which result from genetic polymorphism within the species. Heterogeneity in the V domain distinguishes millions of antibody molecules and forms the basis of antibody diversity.
Antibodies, being proteins, can act as antigens in the same host, resulting in the