Hemolytic disease of the newborn (aka HDNB, Rhesus disease, RH disease, Morbus hemolyticus neonatorum or erythroblastosis) is a condition that occurs when a Rh negative mother has given birth to a Rh positive baby and subsequently becomes pregnant with another Rh positive child. About 5% of at-risk pregnancies would result in still births or extremely sick babies. Many who managed to survive would be severely retarded. Once a woman gives birth to a baby with the disease, all subsequent babies would also have it. The connection between the Rh antigen and erythroblastosis was made in 1941 by Dr. Philip Levine.
During the first pregnancy and the act of birth a small amount of the baby's blood may enter the mother's body. If the mother is Rh negative, her body produces antibodies (including IgG) against the Rhesus antigens on her baby's erythrocytes, if the baby is Rh positive. During the second pregnancy the IgG is able to pass through the placenta into the fetus, where it leads to agglutination and destruction of erythrocytes. The means to prevent this harmful disease is to vaccinate the mother immediately after the birth of her first child: she is treated with anti-Rh antibodies, so that the fetal erythrocytes are destroyed before her immune system can discover them.
Sources: DiaPharma, Wikipedia