Rh Alloimmunization

Rh(D) alloimmunization is an immune-mediated process that begins when Rh(D)-positive fetal red blood cells enter the circulation of an Rh(D)-negative pregnant individual. The exposure may be microscopic: as little as 0.1 mL of Rh(D)-positive red cells can trigger sensitization. The first exposure activates an immune response. Later exposure can produce a rapid memory response with anti-D IgG. Unlike IgM, IgG crosses the placenta. It binds Rh(D) antigens on fetal red blood cells and marks them for destruction, producing fetal hemolytic disease. The clinical problem is therefore not simply that maternal and fetal blood types differ. An Rh-negative person carrying an Rh-negative fetus has no Rh(D)-positive target cells to provoke anti-D formation. Risk arises when the fetus is Rh(D)-positive and fetal cells reach the maternal circulation. Progressive fetal hemolysis causes anemia. Severe anemia can lead to increased extramedullary hematopoiesis, hepatosplenomegaly, hydrops fetalis, and high-output cardiac failure. The newborn may then develop anemia and rapidly rising unconjugated bilirubin after birth. Rho(D) immune globulin, or RhIG, prevents this process by providing passive anti-D that clears Rh(D)-positive fetal cells from maternal blood before the...