Abstract

If red blood cells possessing the D or Rho antigen (Rh) enter the circulation of an Rh-negative woman she may develop Rh antibodies. This is usually the result of pregnancy, but it may follow a mismatched blood transfusion. About one in six Rh-negative women who have been delivered of an Rh-positive infant are found to have antibodies during a subsequent pregnancy and abortion carries the same risk. Incompatibility of the mother with the fetus in the ABO blood group system tends to protect the mother from Rh immunisation, for the mother’s anti-A or anti-B antibody reacts with any fetal A or B Rh-positive cells which may pass into the maternal circulation, so virtually confining them to organs or tissues where antibody formation is unlikely.¹