Which immunoglobulin is the only one that can cross the placenta?

Which immunoglobulin is the only one that can cross the placenta?

• A. IgG
• B. IgM
• C. IgA
• D. IgD

Answer: (A)

IgG is the only immunoglobulin that crosses the placenta because the placenta has specialized Fc receptors (neonatal Fc receptors, FcRn) that selectively bind the Fc portion of IgG and transport it from mother to fetus. This receptor-mediated transfer occurs in fetal circulation and provides the newborn with passive immunity during early life. Other immunoglobulins don’t cross in meaningful amounts. IgM is a large pentamer, which makes it too big to traverse the placental barrier. IgA is mainly secreted across mucosal surfaces and does not use this placental transport pathway. IgD is not produced in a form that is transported across the placenta in significant amounts. This selective transfer explains why maternal IgG antibodies can protect the newborn after birth, and why maternal vaccination boosts the infant’s passive immunity through increased IgG this way.

IgG is the only immunoglobulin that crosses the placenta because the placenta has specialized Fc receptors (neonatal Fc receptors, FcRn) that selectively bind the Fc portion of IgG and transport it from mother to fetus. This receptor-mediated transfer occurs in fetal circulation and provides the newborn with passive immunity during early life.

Other immunoglobulins don’t cross in meaningful amounts. IgM is a large pentamer, which makes it too big to traverse the placental barrier. IgA is mainly secreted across mucosal surfaces and does not use this placental transport pathway. IgD is not produced in a form that is transported across the placenta in significant amounts.

This selective transfer explains why maternal IgG antibodies can protect the newborn after birth, and why maternal vaccination boosts the infant’s passive immunity through increased IgG this way.