The body's immune system is responsible for protection against pathogens—bacteria, viruses, and other microorganisms—that cause infection. Most of the time, the immune system—made up of special cells, tissues, and organs—is efficient at either keeping foreign invaders out or tracking them down and eliminating them inside the body. However, some pathogens, especially ones the body does not recognize, can overwhelm the immune system and cause major problems. Vaccination is a way to "teach" the immune system how to recognize and eliminate an organism, should it find its way inside the body.