When does HDN occur?

• A. During the mother's first pregnancy due to prior sensitization
• B. The mother's antibodies attack and destroy fetal RBCs
• C. Only after delivery
• D. Because the placenta blocks antibody transfer entirely

Answer: (B)

HDN happens when maternal IgG antibodies cross the placenta and attack fetal red blood cells that present a foreign antigen (most commonly the RhD antigen in an Rh-incompatible pregnancy). After sensitization—often from a first exposure to fetal Rh-positive cells during a prior pregnancy or transfusion—the mother makes anti-D antibodies. These circulating antibodies cross the placenta in later pregnancy and bind to fetal RBCs, marking them for destruction by the fetal immune system. This hemolysis leads to fetal anemia and can cause jaundice after birth, and in severe cases hydrops fetalis. The key idea is that the attack is driven by maternal antibodies targeting fetal RBCs, not by the placenta blocking antibody transfer or by destruction occurring only after delivery.