Rh factor, which refers to rhesus monkey and in which these antigens were first found, is another group of antigens discovered in most red blood cells. People are said to be Rh positive if they have these antigens, of course those persons who do not have are Rh negative. For that this condition is recessive to Rh positive, so there are fewer Rh negative persons, nearly 15 percent of the U.S. population. When Rh mothers give birth to Rh positive babies, the Rh factor is especially important.
The Rh negative mother is not commonly exposed to the Rh antigen of the fetus during the pregnancy for that the fetal and maternal blood are usually stained divided in the uterus. Whereas, a variable degree of exposure probably happen at the moment of birth, and the mother’s immune system perhaps become sensitized and antibodies are formed against the Rh antigen. For the reasons that Rh negative women vary in their sensitivity to the Rh factor, and the exposure is probably minimal, so that this condition does not usually happen. Whereas, antibodies can cross the placenta in subsequent pregnancies and lead hemolysis (destruction) of the Rh positive red blood cells of the fetus if the women does form antibodies against the Rh factor. With a condition named erythroblastosis fetalis, or hemolytic disease of the newborn, thus the baby is born anemic.