The humoral immune response elicits the production of immunoglobulins of different structures and biologic functions. These differences are intrinsic to the various classes (isotypes), subclasses, and allotypes of the antibodies made. The binding of antibodies to their respective specific antigens occurs because of physicochemical interactions between tertiary structures of the antibody and the target epitope. Understanding the nature of these interactions allows one to better understand binding affinity and the ability of an antibody made as part of a specific response to one antigen to bind to other, even unrelated, antigens. This ability of an antibody made during the immune response to an infection to bind to different antigens provides the plasticity needed to provide protection from different related or unrelated pathogens.Thus, antibodies made during the response to one adenovirus may be of assistance in the response to infection with a second adenovirus. Cross-reactivity between molecules in and on disparate organisms, e.g., different Gram-negative bacilli, is common; heat-shock proteins and flagellins are very similar and broadly cross-reactive. Antibodies made to one bacterium can be the basis of immune protection from another.The affinity of an antibody for its specific antigen is enhanced during the secondary immune response, because the interaction of the antigen with surface immunoglobulin on memory cells selects the production of antibodies with a "better fit" for the target antigen.In a previous article in this series, on the humoral immune response, we reviewed much of the basic terminology of immunology (epitopes, paratopes, etc.), and described many of the characteristics of antigens and immunoglobulins. In this article we proceed with a description of the various immunoglobulin classes (also known as isotypes) and some of their functional characteristics.