Rh Alloimmunization

Rh(D) alloimmunization is an acquired maternal immune response. It can occur when Rh(D)-positive fetal red blood cells enter the circulation of an Rh(D)-negative pregnant individual. Even a very small exposure—approximately 0.1 mL of Rh(D)-positive red blood cells—may be enough to trigger sensitization. The maternal immune system then forms anti-D antibodies. Those antibodies matter most in a later pregnancy with an Rh(D)-positive fetus: maternal anti-D IgG crosses the placenta, attaches to fetal red blood cells, and accelerates their destruction. The result may be haemolytic disease of the fetus and newborn (HDFN), with fetal or neonatal anaemia and excessive bilirubin production. An Rh-negative blood type does not mean that sensitization has already occurred. It identifies the opportunity for sensitization. A negative antibody screen identifies that clinically significant antibodies have not been detected at that point. Rho(D) immune globulin, usually called RhIG, supplies temporary passive anti-D that clears fetal Rh(D)-positive cells before the maternal immune system mounts its own response. It prevents sensitization; it cannot remove established immune anti-D.