Rh System

An antigen that is sometimes present on the surface of red blood cells is the Rh factor, named after the rhesus monkey in which it was first discovered. Eighty-five percent of the United States' population is Rh-positive (Rh+), meaning that Rh antigens are present. People who do not have Rh antigens are called Rh-negative (Rh).

If an Rh~ person receives a transfusion of blood that has Rh+ antigens, antibodies may react with the antigen and agglutination will occur. The most serious problem with Rh incompatibility occurs during pregnancy. If the mother is Rh~ and the father is Rh+, the child may inherit the dominant Rh+ allele from the father. During delivery, a small amount of the fetus's Rh+ blood may reach the mother's bloodstream. If this happens, the mother will develop antibodies to the Rh factor. If a second Rh+ child is conceived later, the mother's antibodies can cross the placenta and attack the blood of the fetus. This condition is called erythroblastosis fetalis. The fetus may die as a result of this condition, or if the child is born alive, he or she may need an immediate transfusion of Rh+ blood.

To prevent this condition, an Rh~ mother of an Rh+ child can be given antibodies to destroy any Rh+ cells that have entered her bloodstream from the fetus. The mother is, in effect, immunized against the Rh antigen before her immune system has a chance to develop its own antibodies. The antibody treatment prevents Rh sensitization in Rh~ women only if their bodies have not already produced Rh antibodies. If an Rh~ mother has not yet been sensitized, she receives the antibody treatment in the 28th week of pregnancy and again immediately after delivery.

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