Antibody-Antigen Interactions and Measurements of Immunologic Reactions

Antibody-antigen interactions are in many respects comparable to enzyme-substrate interactions (immunologic reactions), in that both are highly specific, reversible, and based on noncovalent intermolecular interactions. The lock-and-key model describes antibody-antigen binding as the perfect fit of two rigid, complementary shapes. An important implication of the induced-fit model is a reduction in antibody-antigen specificity, since the ability of an antibody to subtly change the shape of its antigen-binding site might allow the site to bind multiple antigens. This could explain the frequently observed phenomenon of cross-reactivity, in which an antibody originally generated to one antigen is also capable of binding other, unrelated antigens. Affinity is one of the most important concepts that define the antibody-antigen interaction. The reversibility of antibody-antigen interactions implies that the binding conferred by the noncovalent forces generally is very weak and highly dependent on the distance between the partners. Electrostatic forces are the strongest of all noncovalent bonds but are relatively uncommon in naturally occurring antibody-antigen interactions. Tests based on antigen-antibody interactions are used widely in many fields because of their sensitivity, simplicity, and universal application of the concept that antibodies bind tightly and specifically to antigens. Large variety of laboratory techniques for determining antigen-antibody reactions and cellular responses to antigens have been developed, and sensitive analyses of single cell responses of immunologic relevance are now routinely made.