Which immunoglobulin is transported across the placenta to provide passive immunity to the fetus, and what receptor mediates this transfer?

• A IgM via the neonatal Fc receptor (FcRn)
• B IgE via the high-affinity IgE receptor (FceRI)
• C IgA via the polymeric immunoglobulin receptor
• D IgG via the neonatal Fc receptor (FcRn) ✓

Explanation

Only IgG crosses the placenta, transported through syncytiotrophoblast cells by the neonatal Fc receptor (FcRn), which binds IgG in acidic endosomes and releases it at physiological pH on the fetal side. This transfer begins around the second trimester and gives the fetus passive immunity against tetanus and other pathogens. IgM is too large and never crosses the placenta, its presence in fetal blood suggesting intrauterine infection. The polymeric Ig receptor transports dimeric IgA across mucosal epithelium, not the placenta.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.