Which antibody class crosses the placenta to provide passive immunity to the fetus/neonate?

Which antibody class crosses the placenta to provide passive immunity to the fetus/neonate?

• A. IgA
• B. IgM
• C. IgG
• D. IgE

Answer: (C)

The key idea is that only one antibody class is actively transported from mother to fetus to provide passive immunity: IgG. This transfer happens through the placental Fc receptor (FcRn), which binds the Fc portion of IgG in maternal blood, shuttles it across placental cells, and releases it into fetal circulation. This gives the newborn maternal antibodies that help protect against infections for the first months of life. Other antibody classes don’t cross the placenta effectively. IgM is a large pentamer and cannot pass the placental barrier. IgA mainly protects mucosal surfaces and is supplied to the newborn mainly through breast milk in secretory form, not by placental transfer. IgE is not transferred across the placenta either and is involved in allergy and parasite defense at mucosal surfaces. So, IgG is the antibody class responsible for passive immunity acquired in utero and continued after birth, making it the correct choice.

The key idea is that only one antibody class is actively transported from mother to fetus to provide passive immunity: IgG. This transfer happens through the placental Fc receptor (FcRn), which binds the Fc portion of IgG in maternal blood, shuttles it across placental cells, and releases it into fetal circulation. This gives the newborn maternal antibodies that help protect against infections for the first months of life.

Other antibody classes don’t cross the placenta effectively. IgM is a large pentamer and cannot pass the placental barrier. IgA mainly protects mucosal surfaces and is supplied to the newborn mainly through breast milk in secretory form, not by placental transfer. IgE is not transferred across the placenta either and is involved in allergy and parasite defense at mucosal surfaces.

So, IgG is the antibody class responsible for passive immunity acquired in utero and continued after birth, making it the correct choice.