The immune system consists of white blood cells that come in about 10 million different types. Each type has a protein lock on it called an “antibody,” which corresponds to a key carried by a bacterium called an “antigen.” If a key enters that lock, the white cell starts multiplying ferociously in order to produce an army of white cells to gobble up the key-carrying invader, be it a flu virus, a tuberculosis bacterium, or even the cells of a transplanted heart.
