A neonate born to a mother with autoimmune thrombocytopenia is protected against maternal platelet antibodies despite significant transplacental antibody traffic. Which property of placental immunoglobulin transfer explains selective protection?

• A Secretory IgA crosses via the polymeric immunoglobulin receptor on the villous surface
• B All immunoglobulin classes cross freely by simple diffusion through syncytiotrophoblast gaps
• C IgM crosses preferentially because its pentameric structure binds trophoblast receptors avidly
• D Only monomeric IgG crosses by receptor-mediated transcytosis involving FcRn; other immunoglobulin classes are excluded ✓

Explanation

Maternal IgG alone is transported across the syncytiotrophoblast by receptor-mediated transcytosis using the neonatal Fc receptor (FcRn), conferring passive immunity to the fetus and neonate. IgM is pentameric and far too large for this process, which is why detection of fetal-specific IgM implies active fetal infection rather than maternal transfer. IgA does not cross the placenta and reaches the neonate through colostrum instead.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.