Rhesus incompatibility
Rhesus (Rh) incompatibility is a condition that occurs during pregnancy when a pregnant person is Rh-negative and the fetus is Rh-positive. If the first fetus is Rh-positive, some of the fetal blood can come into contact with the pregnant person’s blood, especially during delivery. The immune system recognizes the fetal blood as foreign and begins to produce antibodies against the Rh antigen.
Rh incompatibility only starts to cause problems in subsequent pregnancies, if the fetus is Rh-positive. The Rh antibodies produced by the pregnant person's immune system can pass through the placenta and attack the fetus’s red blood cells. This results in hemolytic anemia, in which the red blood cells are destroyed faster than the body can regenerate them. Because the red blood cells carry oxygen to all parts of the body, having low red blood cell count could be fatal to the fetus. This condition is also called hemolytic disease of the newborn (HDN).
Treatment
Rho(D) Immune Globulin (Human), or RhoGAM®, is a solution containing antibodies that attach to the Rh-positive blood cells. In this case, the immune system does not react to the Rh-positive cells of the fetus. Consequently, no Rh antibodies are produced by the pregnant person. The dosage administered is low enough not to harm the fetus, and high enough to prevent the pregnant person's immune system from producing antibodies against the Rh factor.