Alloimmunization typically occurs when an individual is exposed to red blood cell (RBC) antigens that are not present in their own blood. The most common scenario is during a blood transfusion where the donor's RBC antigens differ from those of the recipient. Additionally, during pregnancy, if a fetus inherits blood group antigens from the father that are different from the mother's, the mother's immune system may produce antibodies against those fetal antigens. This can result in hemolytic disease of the fetus and newborn (HDFN).
