Rh Factor

Rh Factor, term applied to any of the 30 or more substances, called agglutinogens, found on the surfaces of red blood cells. They are distinct from the main blood types, but their composition is unknown. The Rh factors were named by the American pathologists Karl Landsteiner and Alexander Solomon Wiener, who discovered the first of them in the blood of the rhesus monkey in 1937. This first Rh agglutinogen, corresponding to the one now designated Rh₀, is present in the blood of about 85 percent of all humans. Blood reactions involving Rh factors are now chiefly of interest in obstetrics.

The presence of Rh factors in the blood is controlled by the laws of heredity. An individual who possesses one gene for the Rh factor will express the factor on the red blood cells. If a woman is Rh-negative, that is, if she has two recessive genes for the Rh₀ factor, and a man has two genes that express the Rh-positive factor, then all of their children will be Rh-positive. But if the Rh-positive man has one recessive gene, then each child has a 50 percent chance of being Rh-positive. When carrying an Rh-positive child, the mother will build up antibodies (see Antibody) to the Rh₀ factor in about 5 percent of all cases. These antibodies will usually be too weak to harm the first child. But during labor and delivery some of the baby's Rh-positive blood may get into the mother's bloodstream and trigger or sensitize her immune system. Her antibodies will then attack the red blood cells of any subsequent Rh-positive children. This reaction produces erythroblastosis fetalis, or Rh disease, which results in jaundice, anemia, brain damage, and often death, either before or shortly after birth.

Previously, Rh disease was treated by transfusing the blood of infants who survived. Now most women and men are tested for Rh₀ factor before having children. When an Rh-negative woman has a child by an Rh-positive man, she is given an injection of a substance called Rhogam immediately after the birth. Rhogam is gamma globulin containing antibodies against the Rh₀ factor. By killing any of the Rh-positive red cells that have migrated into the mother's circulation from the fetus, Rhogam prevents the mother from building up immunity to the Rh₀ factor. Thus, subsequent Rh-positive children are not exposed to anti-Rh₀ antibodies. This procedure has largely eliminated Rh disease.

See also Blood.