The antibody anti-Rho(D) causes severe hemolytic disease of the newborn. About 15% of whites and much lower proportions of blacks and Asians are Rho(D)–negative. If an Rho(D)–negative woman carries an Rho(D)–positive fetus, antibodies against Rho(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 Rho(D) immune globulin, a purified concentrate of antibodies against Rho(D) antigen. The Rho(D) immune globulin (one vial of 300 mcg intramuscularly) is given to a Rho(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-Rho(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 Rho(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, Rho(D) immune globulin is no longer helpful and should not be given.