(1) An antigen that may or may notbe present on the surface of human bloodcells. If a person's blood has the antigen, their blood type ispositive, if they do not, it is negative. The rh factor isimportant mainly because if a woman who is Rh- conceives a child who is Rh_, themixing of their bloods in the placenta may provoke an immune reaction in the mother that can cause a life-threatening agglutination of the foetus'blood cells.The Rh factor is so named because it was first identified in rhesus monkeys.
(2) An antigen found in the red blood cells of most people: those who have Rh factor are said to be Rh positive (Rh+), while those who do not are Rh negative (Rh-) Blood used in transfusions much match donors for Rh status as well as for ABO blood group, as Rh- patients will develop anemia if given R+ blood. Rh typing is also important during abortion, miscarriage, pregnancy, and birth, as mother and fetus may not be Rh-compatible. Rh stands for rhesus monkeys, in whose blood this antigen was first found.
See also RhoGAM, Rh incompatibility.