19-04: Prevention of Rhesus Alloimmunization

The antibody anti-Rh_o(D) causes severe hemolytic disease of the newborn. About 15% of people of European and North American descent and much lower proportions of people from Africa and Asia are Rh_o(D)–negative. If an Rh_o(D)–negative woman carries an Rh_o(D)–positive fetus, antibodies against Rh_o(D) may develop in the mother when fetal red cells enter her circulation during small fetomaternal bleeding episodes in the early third trimester or during delivery, abortion, ectopic pregnancy, placental abruption, or other instances of antepartum bleeding. This antibody, once produced, remains in the woman's circulation and poses the threat of hemolytic disease for subsequent Rh-positive fetuses.

Passive immunization against hemolytic disease of the newborn is achieved with Rh_o(D) immune globulin, a purified concentrate of antibodies against Rh_o(D) antigen. The Rh_o(D) immune globulin (one vial of 300 mcg intramuscularly) is given to a Rh_o(D) negative mother within 72 hours after delivery (or spontaneous or induced abortion or ectopic pregnancy). The antibodies in the immune globulin destroy fetal Rh-positive cells so that the mother will not produce anti-Rh_o(D). During her next Rh-positive gestation, erythroblastosis will be prevented. An additional safety measure is the routine administration of the immune globulin at the 28th week of pregnancy to all Rh_o(D) negative mothers. The passive antibody titer that results is too low to significantly affect an Rh-positive fetus. The maternal clearance of the globulin is slow enough that protection will continue for 12 weeks. Once a woman is alloimmunized, Rh_o(D) immune globulin is no longer helpful and should not be given.