Which antibody type is most prevalent in the body and can cross the placenta?

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IgG is the antibody type most abundant in the bloodstream and the only one that can cross the placenta in meaningful amounts. Its transfer from mother to fetus occurs through the neonatal Fc receptor (FcRn) on placental cells, providing the fetus with passive immunity during development. This class of antibody makes up the majority of circulating antibodies, supporting functions like neutralizing pathogens and tagging them for destruction (opsonization), with some ability to activate complement.

IgA mainly protects mucosal surfaces and is abundant in secretions, not the bloodstream, and it does not cross the placenta. IgM is a large pentamer produced early in responses and cannot cross the placenta well. IgE is present in much smaller quantities and is involved in allergic responses and defense against parasites; it does not cross the placenta in significant amounts.