During the last three months of pregnancy some antibodies are passed along the placenta from the mother to the unborn baby.
Antibodies are cells produced by the immune system to defend the body against infection and disease.
Passive immunity is when the baby is given antibodies rather than making them itself. For example, if the mother has had chickenpox, antibodies against chickenpox are passed to the baby via the placenta and the baby has some immunity. If the mother has never had chickenpox, the baby won't be protected against it.
Immunity in newborn babies is only temporary and decreases after a few weeks. Antibodies are also transferred to babies in breast milk, so babies who are breastfed have passive immunity for longer. The thick, yellowish milk (colostrum) that is produced for the first few days after birth is particularly rich in antibodies.
Premature babies are at higher risk of disease because they have received less antibodies so their immune systems aren't as strong.
Because newborn immunity is only temporary, it is important to begin childhood immunisations at two months. This applies whether the baby is premature or full term. The first immunisation includes whooping cough and Hib, because immunity to these diseases decreases the fastest. The jab for measles, mumps and rubella (MMR)is given at around 13 months because passive immunity to these diseases decreases after about a year.