Evolution of Blood Group Antigen Polymorphism

Abstract

Three characteristics define a substance as a blood group antigen. First, the substance is present in the blood, most commonly on the blood cells, but often also in the blood plasma. Substances present exclusively in the plasma and other body fluids are usually not included in the category of blood group antigens. Of the blood cells, the erythrocyte is generally the type locality, the site at which an antigen was originally detected. Although leukocyte and platelet antigens are not excluded from the blood group antigen category, their coverage is usually relegated, both for historical and practical reasons, to separate compendia. Historically, erythrocyte antigens were the first to be described, the existence of leukocyte and platelet antigens was reported much later. The leukocyte antigens — now largely assigned either to the major histocompatibility complex (Mhc) or the cluster of differentiation (CD) series — have become so numerous and their characterization so extensive that justice can be done to them only by separating them from the erythrocyte antigens. Second, to qualify as a blood group antigen, a substance must be capable of eliciting an immune response, usually of the humoral type. Some blood group antigens (e.g., those of the ABO, H, Lewis systems) can probably induce only humoral responses, whereas others (e.g. those of the Rh, MNS, Luteran, Kell, Duffy, Kidd, Diego, and other systems) have at least the potential of also eliciting T lymphocyte-based cellular responses. The prevailing reliance on the humoral response has dictated the use of techniques based almost exclusively on antibody reactions. Serology remains to this day emblematic of blood group antigen studies, although in the future it will probably be replaced either entirely, or at least to a large extent, by molecular biology techniques. Third, the genes encoding blood group antigens are polymorphic so that some individuals of a species possess one form of the antigen and others another. This characteristic is the necessary condition for elicitation of immune response against the antigen which is commonly of the allogeneic type. The blood group antigens are, with few exceptions, alloantigens.

This concluding chapter of the blood group antigen part of this book draws heavily on the information described in the preceding contributions. In the interest of brevity, references have been restricted to a minimum; for complete documentation of the discussed findings, the reader is referred to the relevant chapters.