The immune system is composed of special cells, proteins, tissues and organs which together are capable to mount a well orchestrated attack on any germs that try and invade the body. Your body's Immune System is pretty complicated and sometimes things can go wrong. When this happens, chances are you will be exposed to microorganisms that can result in infection and any kind of illness.
The cells that form part of your immune system are white blood cells or leukocytes. These cells will seek out and destroy organisms that cause disease and they are produced and stockpiled away in multiple spots throughout your body.