Since you have to build an immune system, how is it that newborn babies don't get extremely sicl when they are born?

Renee Neltner
As it turns out, "During the last three months of pregnancy, antibodies from the mother are passed to her unborn baby through the placenta". This is actually a process called passive immunity because of the fact that it is not the baby's own system that is making the antibodies, but they are basically given to the baby's immune system so that they don't get sick immediately after birth. Although this protects the baby as a newborn, the given antibodies start to die off and allow the baby to start making an immune system of its own.

The antibodies that start to die off in a few months can be preserved though if the baby is being breastfed. Breast mil contains additional antibodies and other infection-fighting cells.

So, although newborn babies have not built up their own immune system yet, they are able to fight off infection because of the antibodies given to the baby before it is born.

Sources:
http://www.nhs.uk/chq/Pages/939.aspx?categoryID=54&SubCategoryID=135
http://www.babyzone.com/baby/breastfeeding/newborn-immune-system_65506