In blood banking, an antibody formed in response to pregnancy, transfusion, or transplantation targeted against a blood group antigen that is not present on the person’s red blood cells. These antibodies are most commonly formed against antigens from blood groups such as Rh (including common antigens D, C, c, E, and e), Kell, Kidd, and Duffy (to name a few, but not all by any stretch). The process of forming an alloantibody is called “alloimmunization.” Alloantibodies differ from autoantibodies, which target antigens present on the person’s own red blood cells. In addition, most people do not refer to naturally occurring ABO antibodies (anti-A, anti-B, anti-A,B) as alloantibodies but as “isohemagglutinins.”

Making an alloantibody is important, but only certain antibodies can actually cause clinical harm to a patient. We call such antibodies “clinically significant,” because they may either lead to destruction (“hemolysis“) of transfused red blood cells OR because they may harm a fetus or newborn when a mother carries an alloantibody against an antigen on the baby’s red cells.