How the Body Works: The Rh Factor

There are many different blood groups of which the two most important are the ABO and the Rhesus or Rh groupings. More than 85 percent of the world's population possess the Rh antigen in their blood---and are considered Rh positive. In all other individuals the antigen is lacking and they are classified as Rh negative. Should the antigen enter the bloodstream of an Rh negative person, serious consequences follow. If a Rh negative woman is impregnated by a Rh positive man, the fetus may be Rh positive, too. If the Rh antigen enters the maternal bloodstream via the placenta, it induces the woman to produce anti-Rh antibodies, which could attack subsequent Rh positive fetuses, causing jaundice and possibly death. Treatment in such cases is to transfuse the baby immediately after birth with Rh negative blood.