alloantigene

Alloantigen is an antigen that is present in one individual of a species but absent in another, and can provoke an immune reaction when tissues, cells, or organs bearing the antigen are transferred between individuals. Alloantigens arise from genetic polymorphisms and are especially relevant in transplantation, transfusion medicine, and pregnancy.

The best-known alloantigens in humans are products of the major histocompatibility complex, the human leukocyte antigen (HLA) proteins. Differences in HLA class I and class II molecules between donor and recipient can trigger T cell–mediated rejection. In bone marrow or stem cell transplantation, even HLA-matched donors can cause graft-versus-host disease due to minor histocompatibility antigens, which are polymorphic peptides presented by self-HLA molecules.

Other alloantigens include the ABO blood group antigens and the RhD antigen. Transfusion of blood products with incompatible ABO or RhD antigens can provoke alloimmune responses, leading to hemolytic reactions or alloimmunization against future transfusions. Additional clinically important alloantigens include Kell, Duffy, Kidd, and MNS systems.

Alloantigen recognition involves T cells detecting donor alloantigens presented by recipient antigen-presenting cells or donor cells, and B cells producing alloantibodies. Sensitization can occur during pregnancy, transfusion, or previous transplantation, leading to clinically significant immune responses upon re-exposure.

Clinical implications include consideration of HLA matching, crossmatching, and antigen matching to reduce rejection risk; immunosuppressive therapy in transplant recipients; leukoreduction and antigen-matching transfusion practices to prevent alloimmunization.