Which antibody provides protection to the growing fetus because it is able to cross the placenta?

• A. IgG
• B. IgA
• C. IgM
• D. IgD

Answer: (A)

Maternal antibodies that protect the fetus are those that can cross the placental barrier, and this capability is specific to immunoglobulin G. The placenta expresses the neonatal Fc receptor (FcRn) which binds IgG and ferries it from maternal blood into fetal circulation, providing passive immunity to the developing baby. This transfer helps protect the newborn in the early months of life when its own immune system is still maturing. The other antibody classes do not cross the placenta in meaningful amounts: IgM is too large to pass through, IgA is mainly secreted onto mucosal surfaces and delivered to the infant via breast milk rather than through the placenta, and IgD is mostly found on B cell surfaces with little circulating protective role in this context. Thus, IgG is the antibody that provides fetal protection by crossing the placenta.