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Alloimmunization occurs when a patient is exposed to foreign red blood cell antigens, and subsequently develops antibodies against one or more foreign antigens. This exposure most commonly occurs through pregnancy, blood transfusion or shared needles. For patients without childbearing potential or who are not currently pregnant, the only clinical impact of this antibody development is the patient’s risk for a hemolytic transfusion reaction. It is for this reason that the presence of antibodies is usually discovered during blood group matching in anticipation of transfusion or in early pregnancy during standard first trimester screening.
Clinically significant antibodies have the potential to lyse fetal and newborn red blood cells (RBCs) causing mild to severe hemolysis. This can result in hemolytic disease of the fetus and newborn (HDFN) presenting from mild anemia and icterus to hydrops fetalis, a condition leading to edema and heart failure in the fetus. Though maternal alloimmunization is a serious and nuanced condition, close monitoring and prompt clinical intervention results in a high survival rate for the child of an alloimmunized woman.