Alloantigeenit

Alloantigeenit, or alloantigens, are antigens that differ between individuals of the same species and can provoke an immune response when transferred from one individual to another. They arise from genetic polymorphism and are of particular clinical importance in transplantation, transfusion, and pregnancy.

In humans, the most immunogenic alloantigens are the human leukocyte antigen (HLA) molecules, which are part of the major histocompatibility complex (MHC). These include class I antigens (HLA-A, -B, -C) and class II antigens (HLA-DR, -DQ, -DP). Besides HLA, many minor histocompatibility antigens (MiHAs) derived from polymorphic proteins can contribute to alloimmune reactions.

Allorecognition can occur through direct pathways, where recipient T cells recognize donor MHC molecules on donor cells, and indirect pathways, where recipient antigen-presenting cells present donor-derived peptides. B cells may produce antibodies against donor HLA, leading to antibody-mediated rejection.

Clinical relevance is most pronounced in solid organ and hematopoietic stem cell transplantation. HLA matching reduces rejection risk in organ transplantation, while mismatches can trigger acute rejection. In bone marrow transplantation, donor T cell reactivity against the recipient or vice versa can cause graft-versus-host disease or graft failure. In transfusion medicine, anti-HLA antibodies can cause refractoriness to platelet or other transfusions.

Pregnancy can also lead to alloimmunization when a mother is exposed to paternal alloantigens expressed by the fetus, which may influence future pregnancies or transplant compatibility.