The Rh system is based on the presence or absence in the blood of several factors, the most important of which is a substance called D antigen. Rh-positive blood contains D antigen, whereas Rhnegative blood does not. The blood type is determined by genes.Rhesus incompatibility results if a Rhnegative woman is exposed to the blood of her Rh-positive baby while it is being born. There are usually no problems during the first pregnancy with a Rh-positive baby. However, the woman may produce antibodies against the D antigen; in a subsequent pregnancy with a Rh-positive baby, these antibodies may cross the placenta and attack the red blood cells of the fetus. A Rhnegative woman can also be sensitized if she has had a miscarriage, abortion, or amniocentesis, in which the fetus’s Rhpositive blood enters her circulation.
Rhesus incompatibility is now uncommon because injections of anti-D(Rh0) immunoglobulin are given routinely to Rh-negative women during pregnancy and at delivery. They are also given after miscarriage, abortion, amniocentesis, or any procedure that might result in exposure of the mother to fetal blood cells.
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