Which immunoglobulin is capable of crossing the placenta to provide passive immunity to the fetus?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (A)

The antibody that can cross the placenta to provide passive fetal immunity is IgG. This transfer happens through the neonatal Fc receptor (FcRn) in the placenta, which binds the Fc portion of IgG and moves it from mother to fetus, especially in the third trimester. This gives the newborn systemic protection after birth until its own immune system can fully respond. Other immunoglobulins don’t cross in this way: IgA is mainly secreted at mucosal surfaces (and provided to the newborn primarily through breast milk as secretory IgA), IgM is too large to cross the placental barrier, and IgE is not transported across the placenta.