Alloantibody

In blood banking, an antibody formed in response to exposure to 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.

It is also important to note that most clinically significant alloantibodies are formed as a result of exposure to someone else’s red blood cells during a transfusion, pregnancy, or transplant. Some antibodies, such as those in the Lewis, MNS (the M and N part, not the S and s part), or P1PK/GLOB systems, appear to be formed “naturally,” without the need for transfusion, pregnancy, or transplantation (as a result, we call these antibodies, which are usually NOT clinically significant outside of the ABO system, “naturally occurring” antibodies.

Revised and updated by D. Joe Chaffin MD, December 2024 

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