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.