NBME Immunology Practice Test

Maternal IgG is the only antibody class that crosses the placenta. It is actively transported across the placental barrier by the neonatal Fc receptor (FcRn) on placental syncytiotrophoblasts, delivering maternal antibodies to the fetus and providing passive immunity after birth. The other antibody classes stay in the mother or secretions: IgM is a large pentamer and cannot cross the placenta; secretory IgA mainly protects mucosal surfaces and is passed to the newborn primarily through breast milk; IgE does not cross the placenta.