Rh factor: An antigen found on the surface of red blood cells. Red blood cells with the antigen are said to be Rh positive (Rh+). Those without the surface antigen are said to be 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.