IgG is the immunoglobulin that crosses the human placenta, providing passive immunity to the fetus. This process is mediated by the neonatal Fc receptor (FcRn) on placental cells, making option (C) correct for CSIR NET Life Sciences exam questions on immunology.​

Option Analysis

• IgA: Primarily found in mucosal secretions like saliva and breast milk, IgA does not significantly cross the placenta due to its structure and lack of FcRn binding.​

• IgE: Involved in allergic responses and parasite defense, IgE does not cross the placenta on its own; any fetal presence is typically via rare IgG-IgE complexes, not direct transfer.​

• IgG: The only class that efficiently crosses via active FcRn-mediated transcytosis in syncytiotrophoblast cells, peaking in the third trimester to protect newborns.​

• IgM: First responder to infections and pentameric in structure, IgM is too large and lacks the mechanism to cross the placenta.​

Introduction to Placental Antibody Transfer

Only IgG among immunoglobulins crosses the human placenta effectively, transferring maternal antibodies to protect the fetus from infections during early life when its immune system is immature. This selective process ensures passive immunity, crucial for neonatal health and relevant in exams like CSIR NET Life Sciences.​

Mechanism of IgG Transfer

IgG crosses via FcRn receptors on syncytiotrophoblast cells, enabling active transcytosis from maternal to fetal blood, with efficiency increasing after 30 weeks gestation. Subclasses IgG1 and IgG4 transfer best, influenced by maternal IgG levels and placental integrity.​

Why Other Immunoglobulins Fail

• IgA protects mucosal surfaces but lacks placental transport due to poor FcRn affinity.​

• IgE mediates allergies without direct crossing; fetal IgE arises from complexes or fetal production.​

• IgM, being large, signals fetal infections if detected in newborns as it cannot cross.​

Clinical Relevance

Maternal vaccinations boost IgG transfer, protecting against diseases like pertussis; pathological conditions reduce it, heightening newborn risks. This knowledge aids CSIR NET preparation on immunology and fetal immunity.​