Hemolytic disease of the newborn can occur when which circumstance is present?

• A. The infant lacks an antigen the mother possesses
• B. The mother lacks an antigen the infant possesses
• C. The mother has the antigen the infant lacks
• D. The infant has no antigens in common with the mother

Answer: (B)

Hemolytic disease of the newborn happens when maternal antibodies target fetal red cell antigens that the mother herself lacks. If the fetus inherits an antigen that the mother does not have, the mother’s immune system may become sensitized and produce IgG antibodies against that antigen. These antibodies cross the placenta and attack the fetal red cells, causing hemolysis, anemia, and jaundice after birth. If the mother carries the same antigen as the fetus (the mother has the antigen the infant lacks), there is no foreign antigen to provoke this immune response, so HDN does not develop. Hence, the described situation is that the mother lacks an antigen the infant possesses.