An antigen that may or may not be present on the surface of human blood cells. If a person's blood has the antigen, their blood type is "positive"; if they do not, it is "negative." The Rh factor is important mainly because if a woman who is Rh- conceives a child who is Rh+, the mixing of their bloods in the placenta may provoke an immune reaction in the mother that can cause a life-threatening agglutination of the fetus' blood cells. The Rh factor is so named because it was first identified in Rhesus monkeys. See also ABO blood group.