# Neonatal Immunity > Neonatal immunity for MBBS Physiology: transplacental IgG, secretory IgA in breast milk, immune immaturity and India's immunisation timing logic. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/neonatal-immunity-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Neonatal Immunity", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/neonatal-immunity-physiology ## Direct answer Why does a two-month-old almost never catch measles, while the same child at seven months can? Because maternal immunoglobulin G crosses the placenta by active FcRn-mediated transport from about 16 weeks, accelerating steeply in the third trimester, so a term newborn carries antibody titres at or above maternal levels — cover that decays with a half-life of weeks to months and largely disappears by six to nine months. The protection is passive and borrowed: the preterm infant (missing the third-trimester loading dose) starts deficient, and maternal antibody suppresses the infant's response to live vaccines — why measles vaccine waits until around 9 months in India's schedule (12-15 months in low-transmission countries). The newborn's own system is immature on every axis — neutrophils with sluggish chemotaxis, complement at roughly half adult activity, a T-helper-2 skewed repertoire — while secretory immunoglobulin A in colostrum and milk coats the gut against flora and pathogens it is just meeting. Vitamin K injection at birth plugs the one clotting gap the placenta never transported. ## What you must remember - **IgG transfer facts:** FcRn receptor-mediated, minimal before 16 weeks, steep third-trimester rise; term cord IgG equals or exceeds maternal, preterm is proportionally deprived — part of why preterm infection risk is so high. - **IgM does not cross:** maternal IgM against a fetus is impossible through the placenta, so a raised cord IgM indicates intrauterine infection (TORCH) — a classic interpretive point. - **Breast milk armour:** secretory IgA (highest in colostrum, grams-per-litre concentrations), lactoferrin sequestering iron, lysozyme, oligosaccharides blocking adhesion, and leucocytes — the mucosal shield that formula cannot reproduce, basis of exclusive breastfeeding for six months per WHO and Indian policy. - **Cellular immaturity:** neonatal neutrophils show reduced chemotaxis and reserve, natural killer cells are less cytotoxic, complement and toll-like receptor responses are blunted, and antigen presentation is immature — the physiology behind neonatal sepsis presenting with temperature instability rather than florid signs. - **The susceptibility window:** between waning maternal IgG (gone by roughly 6-9 months) and the infant's full response lies vulnerability to encapsulated bacteria and measles — which immunisation timing narrows. - **Schedule logic for India:** bacillus Calmette-Guerin, hepatitis B birth dose and oral polio zero dose at birth; pentavalent and rotavirus doses from 6 weeks; measles-rubella first dose at 9 completed months with a second at 16-24 months — timing chosen against maternal-antibody interference and local exposure. - **Vitamin K and the flora:** the newborn gut is sterile, so no bacterial vitamin K2 source exists, and breast milk is poor in it — hence intramuscular vitamin K 1 mg at birth to prevent haemorrhagic disease. ## A sepsis vignette, read immunologically A five-day-old term infant feeds poorly with a temperature of 35.8°C and a bulging fontanelle; the mother's membranes had ruptured 18 hours before birth. Why so subtle? Every immature axis blunts the response: neutrophils that chemotax sluggishly, complement at half strength, and an innate bias against interferon-gamma responses that would generate fever — hence hypothermia rather than fever, lethargy rather than localising signs, and a low threshold for lumbar puncture and empirical antibiotics. Contrast the breastfed peer: sIgA lining the gut against maternal flora and hospital organisms, lactoferrin starving bacteria of iron — why exclusive breastfeeding counts among the strongest defences against neonatal sepsis and necrotising enterocolitis in Indian facility guidance. And when this infant recovers and reaches nine months, the measles vaccine timed then will finally escape the maternal IgG that would have neutralised it at six. ## Where students slip The first slip is calling neonatal immunity "a blank slate": it is a borrowed library plus an immature but functional own system — the difference matters clinically and in vaccine scheduling. Second, students mix up which immunoglobulin crosses the placenta (IgG only) with which dominates milk (secretory IgA) — a one-mark distinction asked relentlessly. Third, the reason for delayed measles vaccination is attributed vaguely to "the baby is too small"; the actual mechanism is neutralisation of live vaccine virus by residual maternal antibody, with the Indian 9-month choice balancing earlier transmission risk against that interference. Fourth, physiological jaundice gets drafted into immune answers — it is a bilirubin enzyme story, not an antibody story. Finally, maternal tetanus antibody transfer is the mechanism behind the tetanus-diphtheria booster of pregnant women, a national programme application worth quoting. ## Frequently asked questions ### Which immunoglobulin crosses the placenta and how? Immunoglobulin G, actively transported by the FcRn receptor with a steep third-trimester rise, giving the term newborn titres at or above maternal levels. ### Why does raised cord IgM suggest intrauterine infection? Maternal IgM cannot cross the placenta, so IgM in cord blood must be the fetus's own response to an antenatal pathogen such as TORCH agents. ### What immune factors does breast milk supply? Secretory IgA, lactoferrin, lysozyme, oligosaccharides and leucocytes — mucosal protection without depending on the infant's immature systemic responses. ### Why is measles vaccine given at 9 months in India? Residual maternal IgG neutralises live vaccine virus earlier; 9 months balances waning maternal antibody against early measles exposure in high-transmission settings. ### Why is vitamin K given at birth? The sterile newborn gut lacks bacterial vitamin K2 synthesis and breast milk is vitamin K-poor, so intramuscular vitamin K prevents haemorrhagic disease of the newborn.