Which immunoglobulin is primarily transferred from mother to fetus providing passive immunity?

• A. IgG
• B. IgM
• C. IgA
• D. IgD

Answer: (A)

The main idea is that maternal antibodies crossing to the fetus to provide temporary protection are mainly the immunoglobulin G (IgG) type. IgG is actively transported across the placenta by neonatal Fc receptors, delivering systemic passive immunity to the fetus and newborn. This transfer occurs predominantly in the third trimester, helping protect the newborn after birth while their own immune system is still developing. Other immunoglobulins don’t cross in meaningful amounts: IgM is too large to cross the placenta and stays in the mother; IgA is largely secreted to mucosal surfaces and is provided to the infant mainly via breast milk after birth; IgD has minimal placental transfer and isn’t a major contributor to fetal immunity. Thus IgG is the antibody that best provides passive fetal immunity before the newborn can mount its own immune response.