Rh factor

Rh factor An antigen found in the red blood cells of most people. Those who have Rh factor are said to be Rh positive (Rh+), and those who do not are Rh negative (Rh-). Blood used in transfusions must match donors for Rh status as well as for ABO blood group because 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 Rh incompatibility; RhoGAM.