If your immune system thought your baby was foreign, it would mount an attack on him. You and your baby are designed so that this does not happen. Your baby does not have the ability to produce antibodies (which would attack you) in the uterus: he relies completely on you to protect him from infection, not only in the uterus but also after birth. Protection after birth is possible because antibodies from your immune system cross over the placenta into your baby's bloodstream, while you're pregnant. If you have immunity to a disease such as measles, mumps, polio, and many other severe infections, your baby will carry your antibodies to these conditions. This so-called passive immunity is lost with time and it is for this reason that, from two months onward, your baby will require a program of immunizations to protect against these and other illnesses.