Isoimmunization is the development of antibodies which are elicited by antigens of another individual of the same species. One clinical example is the Rh isoimmunization of the mother. During the course of pregnancy, fetal red cell fragments are often found in the maternal blood. (about 1/3 of mothers in the third trimester has fetal red cell fragments in their blood). If the mother is Rh negative, and the fetus is Rh positive, the fetal red cells would cause isoimmunization of the mother.