For 9 months, your baby was safe in the sterile environment of the womb. As the fetus was developing, it was building the cells necessary to later become a fully functioning immune system. However, growing in a sterile environment and protected by the placenta, the fetal immune system was turned off while in the womb. Now that your baby is born, they are exposed to pathogens in the environment and their immune system must adapt.
Your newborn’s immune system will fully develop over the course of their first year of life. Like other aspects of a newborn, it takes time to adjust to being out in the world. A baby starts by only eating milk because their gastrointestinal (GI) system does not function like an adult. Similarly their immune system also takes time to mature.
Building Your Baby’s Immune System
Since your newborn’s immune system is relatively inexperienced at first, they can be at increased risk of infection. The good news is that as a baby’s immune system grows, maternal antibodies received during pregnancy protect them. Maternal antibodies are antibodies from the pregnant person that crossed the placenta and are taken up by the fetus. They protect the baby after birth from diseases the pregnant person has encountered or been vaccinated against. This is one reason it is important for people to get certain vaccines during pregnancy. Breastfeeding is another way your baby receives maternal antibodies to support the immune system as it develops.
Your baby will have maternal antibodies for their first 3 to 6 months of life, giving their immune system time to grow strong. During this time your baby’s immune system will start to produce immune cells, like B cells and T cells. These immune cells will start to encounter pathogens and learn how to protect your baby.
In addition to maternal antibodies, immunizations are vital to protect your baby while their immune system grows. Experts recommend immunizations at specific times based on immune system development and the optimal response for each vaccine. Healthcare providers give some vaccines earlier while the immune system is still developing, while others are given later when it is stronger. These decisions are based on decades of scientific research and rigorous clinical trials that have shown when immunizations are best for your baby. This is all outlined in the immunization schedule, which you can learn more about here.