Alloantigen

Alloantigen refers to an antigen that is present in some individuals of a species but absent in others, such that exposure to it from a genetically different individual can provoke an immune response. Alloantigens are central to alloimmune reactions that can occur when tissues, cells, or blood from one person are transferred to another. In humans, important alloantigens include the human leukocyte antigens (HLA, the major histocompatibility complex), ABO and other blood group antigens, and numerous minor histocompatibility antigens.

Alloantigen recognition involves the recipient’s immune system detecting donor alloantigens as foreign. This can trigger antibody-mediated responses and T cell–mediated reactions, leading to inflammation, cell destruction, and tissue injury. The genetic polymorphism that creates variability in HLA, ABO, and other surface proteins underpins the existence of alloantigens between individuals.

Clinically, alloantigens are most consequential in transplantation, transfusion, and pregnancy. In transplantation, mismatched alloantigens can cause hyperacute, acute, or chronic rejection, while in transfusion, alloantigens can provoke hemolytic reactions. In hematopoietic stem cell transplantation, donor alloantigens can contribute to graft-versus-host disease. Maternal-fetal alloimmunization can produce antibodies against fetal alloantigens, sometimes leading to hemolytic disease of the newborn. Management strategies include antigen matching (especially HLA and ABO), cross-matching, and immunosuppressive therapy to reduce alloimmune responses. Alloantigens are distinct from autoantigens, which are self-derived, and from xenogeneic antigens, which originate from different species.