Alloimmunity (sometimes called isoimmunity) is an immune response to nonself antigens from members of the same species, which are called alloantigens or isoantigens. Two major types of alloantigens are blood group antigens and histocompatibility antigens. In alloimmunity, the body creates antibodies (called alloantibodies) against the alloantigens, attacking transfused blood, allotransplanted tissue, and even the fetus in some cases. Alloimmune (isoimmune) response results in graft rejection, which is manifested as deterioration or complete loss of graft function. In contrast, autoimmunity is an immune response to the self's own antigens. (The allo- prefix means "other", whereas the auto- prefix means "self".) Alloimmunization (isoimmunization) is the process of becoming alloimmune, that is, developing the relevant antibodies for the first time.

Alloimmunity is caused by the difference between products of highly polymorphic genes, primarily genes of the major histocompatibility complex, of the donor and graft recipient. These products are recognized by T-lymphocytes and other mononuclear leukocytes which infiltrate the graft and damage it.