A neonate is born with a serum IgG level that is higher than the maternal level. The mechanism by which the placenta transfers IgG from mother to fetus is:

• A Simple diffusion down a concentration gradient
• B Active transport via FcRn receptors on syncytiotrophoblast ✓
• C Facilitated diffusion through gap junctions
• D Pinocytosis of maternal plasma

Explanation

IgG is actively transported across the placenta via neonatal Fc receptors (FcRn) on the syncytiotrophoblast. This receptor-mediated transport begins in the second trimester and accelerates in the third trimester, resulting in fetal IgG levels that exceed maternal levels at term. This passive immunity protects the neonate for the first 6 months of life. IgM and IgA do not cross the placenta significantly because they lack affinity for FcRn.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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