isohemagglutinins

Isohemagglutinins are antibodies directed against isoantigens on red blood cells of individuals with different blood group phenotypes. The classic examples are anti-A and anti-B antibodies that occur in individuals with blood type O. These antibodies can cause agglutination of red cells in the presence of the corresponding antigen, a reaction known as hemagglutination.

Most isohemagglutinins are of the IgM class and are produced naturally without deliberate exposure to foreign red blood cell antigens. They are predominantly cold-reactive, reacting at room temperature or colder, but can participate in transfusion reactions when incompatible blood is given. The antibodies are thought to arise from exposure to environmental antigens—most likely bacteria—that share epitopes with A and B antigens.

Clinically, isohemagglutinins govern ABO compatibility in transfusion and, to a lesser extent, in transplantation. Incompatibility can lead to acute or delayed hemolytic reactions due to antibody-mediated destruction of donor red cells. In pregnancy, maternal isohemagglutinins can cross the placenta and cause hemolytic disease of the newborn in ABO-incompatible cases, though this condition is typically milder than Rh(D) disease.

Laboratory detection includes forward and reverse typing, crossmatching, and titration of isohemagglutinin activity; anti-A and anti-B titers are part of serologic testing in transfusion medicine. The term can also apply to antibodies against other non-ABO red cell antigens (such as anti-M or anti-N) that cause isohemagglutination.