Which immunoglobulin provides the fetus with humoral immunity against tetanus and diphtheria toxins before birth?

• A IgM synthesized by the fetal spleen after antigen exposure
• B IgA secreted into amniotic fluid and swallowed
• C IgG transported across the placenta via trophoblast Fc receptors ✓
• D IgE transferred through the yolk sac circulation

Explanation

Only IgG crosses the placenta, actively transported by FcRn receptors on syncytiotrophoblast, with transfer accelerating in the third trimester so term infants are born with maternal IgG levels equal to or above maternal levels. IgA reaches the neonate later through colostrum and breast milk, acting on the gut mucosa. Fetal IgM is produced only after intrauterine antigenic stimulation such as congenital infection.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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