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Immunology · Adaptive immunity

Neonatal immunity: maternal IgG and breast-milk IgA !

  • Topic

Newborns are protected at first by antibodies from their mother: IgG through the placenta and IgA in breast milk.

Updated 2026-11-01

What it is

The newborn's own adaptive immunity is immature, so early protection is borrowed. Maternal IgG crosses the placenta, mostly in the last weeks of pregnancy, and gives temporary passive immunity that fades over the first months. Breast milk, especially colostrum, supplies secretory IgA and other protective factors that guard the infant's gut and airways. Knowing this explains why preterm infants are vulnerable, why some vaccines are given in pregnancy, and how maternal antibodies can harm the fetus.

Key facts

  • Only IgG crosses the placenta, actively transported by the neonatal Fc receptor (FcRn).
  • Transfer is greatest in the third trimester, so preterm babies receive less IgG.
  • Maternal IgG falls over the first months; the infant's own IgG rises slowly, giving a physiological low point (transient hypogammaglobulinemia) around three to six months.
  • Secretory IgA in colostrum and milk is not absorbed into the blood; it protects the mucosa locally.
  • IgM does not cross the placenta, so specific IgM in a newborn was made by the baby.

Function

  • Maternal IgG protects against infections the mother is immune to, such as measles, tetanus and diphtheria, during early infancy.
  • Breast-milk IgA, lactoferrin, lysozyme and oligosaccharides reduce gastrointestinal and respiratory infections.
  • Vaccination during pregnancy (for example against pertussis, influenza or tetanus) increases the antibodies passed to the baby.

Clinical notes

  • Specific IgM in cord or newborn blood suggests congenital infection (for example toxoplasmosis, rubella, CMV).
  • Maternal anti-D IgG can cause hemolytic disease of the fetus and newborn, with anemia and neonatal jaundice.
  • Maternal IgG antibodies can cause transient neonatal disease, such as neonatal Graves disease, neonatal myasthenia or neonatal lupus with heart block.
  • Maternal antibodies can neutralize some live vaccines, one reason the measles-containing vaccine is given around the first birthday.
  • A positive IgG test in a young infant may reflect maternal antibody rather than infection (for example HIV antibody tests).

Test yourself

Think of the answer, then tap the question to see it.

Which immunoglobulin crosses the placenta, and by what mechanism?

IgG, actively transported by the neonatal Fc receptor (FcRn) in placental cells.

Why are preterm infants more vulnerable to infection in terms of maternal antibodies?

Most maternal IgG transfer happens in the third trimester, so babies born early receive less.

What does specific IgM in a newborn's blood suggest?

A congenital infection, because maternal IgM does not cross the placenta; the fetus made it.

Why can an HIV antibody test be positive in an uninfected infant of a mother with HIV?

Maternal anti-HIV IgG crosses the placenta and can persist for many months; infection in infants is confirmed with virologic (nucleic acid) tests.

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Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: Chapter 1: Principles of Vaccination (Centers for Disease Control and Prevention (CDC), Pink Book, 2021); 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies (OpenStax (Anatomy and Physiology 2e), 2022); 18.1 Overview of Specific Adaptive Immunity (OpenStax (Microbiology), 2016).

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