EXOGENOUS AND ENDOGENOUS ANTIGENS
Most antigens to which we react are of exogenous origin and include microbial antigens, environmental antigens (such as pollens and pollutants), and medications. The objective of the immune response is the elimination of foreign antigens, but in some instances the im-mune response itself may have a deleterious effect resulting in hypersensitivity or in au-toimmune disease.
Antigenic determinants that distinguish one individual from another within the same species are termed “alloantigens.” The alloantigens that elicit the strongest immune re-sponse are alleles of highly polymorphic systems, such as the erythrocyte A, B, O blood group antigens: some individuals carry the polysaccharide that defines the A specificity, while others have B positive red cells, AB positive red cells, or red cells that do not express either A or B (O). Other alloantigenic systems that elicit strong immune responses are his-tocompatibility (MHC or in the human, HLA) antigens of nucleated cells and tissues, the platelet (PI) antigens, and the Rh erythrocyte blood group antigens. Examples of sensitiza-tion to exogenous alloantigens include:
· Women sensitized to fetal red cell antigens during pregnancy
· Polytransfused patients who become sensitized against cellular alloantigens from the donor(s)
· Recipients of organ transplants who become sensitized against histocompatibility al-loantigens expressed in the transplanted organ
Endogenous antigens, by definition, are part of self, and the immune system is usu-ally tolerant to them. The response to self antigens may have an important role in normal catabolic processes (i.e., antibodies to denatured IgG may help in eliminating antigen-anti-body complexes from circulation; antibodies to oxidized low-density lipoprotein (LDL) may help in eliminating a potentially toxic lipid). The loss of tolerance to self antigens, however, can also have pathogenic implications (autoimmune diseases).