Erythroblastosis fetalis is secondary to Rh blood group incompatibility. In this mother is Rh -ve and fetus is Rh +ve. This usually happens not in the first pregnancy, but in subsequent pregnancies.
Rh antigens are C, D and E. Of this D is responsible for incompatible reactions, others do not produce reactions. Those mothers who do not have Rh D antigens are Rh -ve. In Asian populations 99% are Rh+ve and in whites, it is 85%.
Unlike ABO system anti-D antibodies are not formed till they Rh-ve person gets exposed to Rh+ve blood. An Rh -ve person may be exposed to Rh+ve blood either by parental route like blood transfusion by Rh+ve blood or during delivery of Rh+ve baby by Rh-ve mother. The mother gets contaminated by fetal blood during delivery. Following exposure to Rh+ve blood, anti-D antibodies are formed, i.e during the post-partum period. First pregnancy escapes from this Rh incompatibility reaction. Subsequent pregnancy this anti-D antibody crosses the placental barrier and reach the fetus. if that fetus is Rh+ve then erythroblastosis fetalis may occur. This presents with hemolysis in the fetus, if that is severe, the infant may die in utero or may develop anemia, severe jaundice, and edema (hydrops fetalis). Kernicterus, a neurologic syndrome in which unconjugated bilirubin is get deposited in basal ganglia
Prevention is by preventing Rh -ve mothers from developing ant-D antibodies. This is done by passive immunization of mothers with anti-D antibodies during the postpartum period, so the mother does not develop anti-D antibodies actively.
Ref Ganong chapter 31 edition 26