A person exposed to a specific immunizing event may produce “immune” ABO antibodies of the same specificity as the naturally occurring antibody but with different biological behavior. Examples of immunizing events that may produce these ABO antibodies include pregnancy with an ABO-incompatible fetus or transfusion of ABO-incompatible red cells. After immunization, the subject’s antibody may increase in titer and/or avidity, develop powerful hemolyzing properties, or become more active at 37°C.
