Learn how vaccines help the immune system protect people against diseases in this video from NOVA: "Vaccines—Calling the Shots." The immune system is a natural defense system that uses biological processes to fight invaders such as germs. Vaccines introduce a weakened germ to the body so that the immune system learns how to fight it and is prepared for a future invasion. Animations illustrate how white blood cells and antibodies fight germs, and show how vaccines arm the immune system with memory cells. Vaccines prevent disease epidemics and are especially important for babies, whose immune systems are immature and vulnerable to infection.