Which immunoglobulin is efficiently transported across the placenta from mother to fetus?

• A. IgA
• B. IgD
• C. IgG
• D. IgM

Answer: (C)

IgG is the immunoglobulin that crosses the placenta efficiently to give the fetus passive immunity. This transport is carried out by the neonatal Fc receptor (FcRn) on placental cells, which binds IgG and shuttles it from mother to fetus. This mechanism specifically favors IgG, so other antibodies don’t cross in significant amounts: IgM is a large pentamer and can't pass the placental barrier, IgA is mainly produced for mucosal defense and is secreted into breast milk or secretions rather than fetal circulation, and IgD is not a major placental antibody. As a result, newborns receive maternal IgG, providing protection for several months after birth.