Alloantigen

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 antigens. In alloimmunity, the body creates antibodies 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 , of the donor and graft recipient. These products are recognized by T-lymphocytes and other mononuclear leukocytes which infiltrate the graft and damage it.

A failure of cross-matching can allow donor blood of an incompatible blood group to be transfused, resulting in a transfusion reaction.

Hemolytic disease of the fetus and newborn is similar to a transfusion reaction in that the mother's antibodies cannot tolerate the fetus's antigens, which happens when the immune tolerance of pregnancy is impaired. In many instances the maternal immune system attacks the fetal blood cells, resulting in fetal anemia. HDN ranges from mild to severe. Severe cases require intrauterine transfusions or early delivery to survive, while mild cases may only require phototherapy at birth.

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