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rh factor

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.

See: aBO blood group.