During the last 3 months of pregnancy, antibodies from the mother are passed to her unborn baby through the placenta. This type of immunity is called passive immunity because the baby has been given antibodies rather than making them itself. Antibodies are special proteins the immune system produces to help protect the body against bacteria and viruses. The amount and type of antibodies passed to the baby depends on the mother’s immunity. For example, if the mother has had chickenpox, she’ll have developed immunity against the condition and some of the chickenpox antibodies will be passed to the baby. But if the mother hasn’t had chickenpox, the baby won’t be protected. Immunity in newborn babies is only temporary and starts to decrease after the first few weeks or months.
Breast milk also contains antibodies, which means that babies who are breastfed have passive immunity for longer. The thick yellowish milk (colostrum) produced for the first few days following birth is particularly rich in antibodies. Premature babies are at higher risk of developing an illness because their immune systems aren’t as strong and they haven’t had as many antibodies passed to them. As newborn immunity is only temporary, it’s important to begin childhood immunisations when your baby is 2 months old. This applies to babies who are either premature or full-term. The first immunisation, given when your baby is 2 months old, includes whooping cough and Hib (haemophilus influenza type b) because immunity to these conditions decreases the fastest. Passive immunity to measles, mumps and rubella can last for up to a year, which is why the MMR vaccine is given just after your baby’s first birthday. https://www.nhs.uk/common-health-questions/childrens-health/how-long-do-babies-carry-their-mothers-immunity/