Definition of Rh factor

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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+), 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.