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

• A. IgM
• B. IgA
• C. IgG
• D. IgE

Answer: (C)

Crossing the placenta to provide passive immunity is accomplished mainly by IgG. This immunoglobulin is transported across the placental barrier by the neonatal Fc receptor (FcRn) on placental cells, which binds IgG and ferries it into fetal circulation. This transfer provides the fetus and newborn with circulating antibodies for protection during the early months of life. Other immunoglobulins are not significantly transferred through the placenta: IgM is a large pentamer and cannot cross, IgA is primarily secretory and found in mucosal surfaces and breast milk, and IgE is not efficiently transferred. In short, IgG is the antibody class that supplies the fetus with passive immunity before its own immune system is fully functional.