A vaccine contains a specific part

A vaccine contains a specific part of a germ (bacteria or virus), called an antigen. The antigen is killed or disabled before it's used to make the vaccine, so it can't make you sick. Vaccines, and the antigens they contain, stimulate your immune system's B cells to develop protective substances called antibodies. These antibodies are responsible for killing germs that enter your body. Once activated, B cells can stay in your body for a lifetime and allow your body to remember the germ that stimulated their creation. Throughout your life, these cells will recognize and fight the actual disease caused by the germ when and if you come into contact with it.