Dhirendra N Misra Heinz W Kunz and Tl

of Pittsburgh, Pittsburgh, Pennsylvania, USA

J Gill III, Department of Pathology, University

Molecules expressed on biologically active cells include structural components, enzymes, receptors, transport system molecules, viral proteins and accessory molecules. Many of them carry unique determinants that can elicit xenogeneic and/or allogeneic immune responses. Alloantigens play crucial roles in the body's defense mechanisms and are of importance in the clinic with respect to blood transfusion and tissue or organ transplantation. Recent advances in serological and molecular biological techniques and in X-ray crystallographic structural analysis have increased our ability to identify and characterize new cell surface antigens and to understand their structure and function.

Alloantigenic polymorphism can arise in several ways: first, sequence variation in structural genes (e.g. major histocompatibility complex (MHC) antigens, Rh blood group antigens); second, variation in the activity of enzymes involved in the synthesis of antigenic epitopes (e.g. ABH or Lewis blood group carbohydrate determinants); third, sequence variation in regulatory elements controlling the expression of structural genes (e.g. expressed versus null alleles as for H2-T antigens in the mouse).

Alloantigenic responses also can occur in several ways: 1) alloimmunization; 2) natural immunization (e.g. naturally occurring antibodies against ABH antigens); and 3) the maternal immune response against fetal antigens during pregnancy (e.g. antibodies against human MHC antigens). Some major alloantigenic systems are described in the following section and summarized in Table 1.

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