Antibodies are specialized proteins produced by the immune system to identify and neutralize foreign invaders. Because a baby’s immune system is immature, nature provides a temporary defense mechanism called passive immunity. This involves the transfer of pre-made antibodies from the mother, offering the infant immediate, ready-to-use protection. This immunity allows the baby time to safely build its own adaptive immune response against infectious agents encountered after birth.
Antibody Transfer Through the Placenta
The most significant transfer of protective antibodies occurs before birth, directly across the placenta. This prenatal defense is mediated almost exclusively by Immunoglobulin G (IgG). Specialized cells in the placenta actively transport IgG from the maternal bloodstream into the fetal circulation, ensuring the baby is born with a circulating supply of defense proteins.
This transfer process is highly efficient and is mediated by the neonatal Fc receptor (FcRn). The receptor binds to the IgG antibody at an acidic pH, shuttles it across the placental barrier, and releases it into the fetal blood when the pH neutralizes. While this transfer begins early in pregnancy, the rate accelerates dramatically during the third trimester, with the highest concentration occurring between 37 and 40 weeks of gestation.
By the time a full-term baby is born, the concentration of IgG antibodies in its blood often matches or exceeds the concentration found in the mother. This transferred IgG provides systemic protection, circulating throughout the baby’s body to fight off infections in the bloodstream and tissues. Since this supply builds up late in pregnancy, premature infants often have lower initial levels, making them more vulnerable in the first weeks of life.
Postnatal Immunity Via Breast Milk
After birth, the transfer of protective factors continues through human milk, but the type of antibody and its function changes. The dominant antibody in breast milk is Secretory Immunoglobulin A (sIgA), which provides localized immunity. Colostrum, the thick milk produced in the first few days postpartum, is particularly rich in sIgA, with concentrations decreasing as the milk matures.
Unlike the IgG transferred across the placenta, sIgA stays on mucosal surfaces and does not enter the infant’s bloodstream. This localized action allows sIgA to coat the lining of the baby’s gut and respiratory tract, forming a protective barrier. By coating these surfaces, sIgA neutralizes pathogens and toxins, preventing them from adhering and causing infection.
Small amounts of other antibodies, including IgG and Immunoglobulin M (IgM), are also present in breast milk, but the primary function of milk-based immunity is the localized defense provided by sIgA. This ongoing mucosal protection helps guide the colonization of a healthy gut microbiome while actively fighting off ingested or inhaled microbes.
The Duration and Targets of Passive Immunity
The protection afforded by maternal antibodies is temporary, as the baby’s body does not produce them and gradually breaks them down. The IgG antibodies received via the placenta have a half-life of roughly 21 to 23 days, meaning their concentration halves approximately every three weeks. Consequently, the systemic protection provided by this transferred IgG typically wanes by six to eight months of age, and most maternal antibodies are cleared from the infant’s circulation by 12 months.
The benefit of this placental IgG is that it targets any infection the mother has immunity to, whether from past illness or vaccination. This includes protection against serious diseases like measles, tetanus, diphtheria, and pertussis, as well as common viruses such as influenza and respiratory syncytial virus (RSV). This protection covers the first few months of life when the infant is too young to fully respond to its own vaccinations.
In contrast, the localized protection from breast milk sIgA lasts only as long as the baby continues to consume the milk. This sIgA is broken down in the gut after performing its protective function, meaning the defense is renewed with every feeding. The baby’s mucosal surfaces are shielded against pathogens present in the immediate environment, such as those that cause diarrhea or respiratory illness, for the entire duration of breastfeeding.
Maternal Factors Influencing Antibody Quality
The quality and quantity of antibodies transferred to the baby are directly influenced by the mother’s current immune status. Recent exposure to an infection or a vaccine prompts her body to produce high levels of specific, targeted antibodies. These circulating antibodies are then preferentially transferred to the fetus via the placenta and into the breast milk.
Maternal vaccination during pregnancy is a highly effective strategy to enhance passive immunity in the infant. For instance, receiving the Tdap vaccine (tetanus, diphtheria, and pertussis) between 27 and 36 weeks of gestation significantly increases the concentration of pertussis-specific IgG available for placental transfer. This boost provides the newborn with heightened protection against whooping cough during their most vulnerable first months.
Vaccination against influenza and COVID-19 during pregnancy helps ensure the transfer of specific antibodies against these diseases. By strategically vaccinating during the optimal window, the mother maximizes the amount of protective IgG available for transport just before the high-volume transfer phase in the third trimester. This intentional boosting of maternal antibody levels provides a powerful immune advantage to the newborn.