Immune complexes are formed by the interaction of antigen and antibody. Typically, the combination occurs between the antigen and the antibody, the formation of which it specifically elicited. However, in some cases, the antibody may bind to a substance that is structurally similar to the specific antigen. The antigen involved may be exogenous—such as a drug, infectious agent, or ingested food—or endogenous—that is, normal tissue antigen that has been altered so that it becomes immunogenic, as is seen in autoimmune diseases. The antibody involved may belong to any one of the five immunoglobulin classes (IgG, IgM, IgA, IgD, or IgE).
Immune complex formation in general is a regulatory mechanism whereby the immune system eliminates noxious or foreign antigens from the circulation. The complexes are rapidly taken out of the circulation by the cells of the reticuloendothelial system (RES), through a mechanism to be discussed later, and then discarded from the