What is the role of maternal IgG in immunity?

• A. Provides passive immunity to the fetus/neonate by crossing the placenta.
• B. Provides active immunity to the fetus through in utero immune priming.
• C. Stimulates development of the fetal immune system via thymic education.
• D. Induces autoimmunity in the newborn.

Answer: (B)

Maternal IgG provides passive immunity to the fetus by crossing the placenta and entering the fetal circulation. This transfer happens mainly in the third trimester via Fc receptors (FcRn) in the placenta, so the newborn benefits from antibodies the mother already has without having to produce its own response. This is different from active immunity, where the individual generates its own antibodies after exposure to an antigen and can develop memory. The fetus’s immune system isn’t being primed in utero by maternal antibodies to create its own protective memory, and maternal IgG does not induce thymic education or autoimmunity in the newborn. After birth, maternal IgG wanes, and the infant must rely on its own immune responses.