Final answer:
When a mother is sensitized to the Rh(D) antigen, her immune system produces anti-Rh(D) antibodies. This can happen after the birth of an Rh-positive baby to an Rh-negative mother. If the mother has another Rh-positive child, the anti-Rh(D) antibodies can cross the placenta and cause hemolytic disease of the newborn (HDN).
Step-by-step explanation:
When a mother is sensitized to the Rh(D) antigen, it means that her immune system has been exposed to the antigen and has started producing anti-Rh(D) antibodies. This usually occurs after the birth of an Rh-positive baby to an Rh-negative mother, as the baby's Rh-positive blood can enter the mother's bloodstream during pregnancy or delivery.
After sensitization, if the mother conceives another Rh-positive child, the anti-Rh(D) antibodies can cross the placenta and attack the fetal blood, leading to a condition called hemolytic disease of the newborn (HDN) or erythroblastosis fetalis. Severe cases of HDN can result in anemia and even fetal death.