How Shots Work
Even infants produce antibodies, substances that fight off unwelcome bacteria and viruses. A vaccine contains antigens, weakened versions or parts of disease-causing germs; these will prompt your baby's immune system to make antibodies. If a child is ever exposed to the actual germ, these antibodies will attack it before it causes the disease. The result is that your baby develops immunity without having to suffer through the illness or its complications, says Dr. Orenstein.