allorecognition

Allorecognition is the immune system's ability to detect antigens from another individual of the same species that differs genetically. It underpins responses to transplanted tissues and cells, as well as certain blood products, and can lead to graft rejection or graft-versus-host disease. Allorecognition is distinguished from autoimmunity (self antigens) and xenorecognition (antigens from different species).

In humans and other vertebrates, allorecognition is primarily mediated by T cells. Direct allorecognition occurs when recipient T cells recognize intact donor MHC molecules on donor antigen-presenting cells. Indirect allorecognition occurs when recipient antigen-presenting cells present donor-derived peptides in the context of self-MHC to T cells. Minor histocompatibility antigens and non-MHC alloantigens can also contribute, and natural killer cells may participate in certain contexts. B cells may produce alloantibodies against donor antigens, particularly HLA molecules.

Clinical consequences include several forms of graft rejection: hyperacute rejection due to preformed antibodies, acute rejection occurring weeks to months after transplantation, and chronic rejection developing over longer periods. In hematopoietic stem cell transplantation, allorecognition can cause graft-versus-host disease, in which donor immune cells attack recipient tissues.

Applications and management focus on reducing alloreactivity. HLA matching and careful donor selection aim to minimize immune incompatibilities. Immunosuppressive therapy (such as calcineurin inhibitors and steroids) and desensitization strategies are used to prevent rejection, while research explores tolerance induction and mixed chimerism to lessen dependence on immunosuppression.

Allorecognition reflects the immune system’s capacity to distinguish closely related, non-self antigens within a species and is central to transplantation immunology.