When your body is attacked--perhaps by a virus or germs on a nail you stepped on--your immune system defends you. It sees and kills the germs that might hurt you. But when the system doesn't work right, this process can cause harm. Immune cells can mistake your body's own cells as invaders and attack them. This "friendly fire" can affect almost any part of the body. It can sometimes affect many parts of the body at once. This is called autoimmunity (meaning self-immunity).

Immune System/AIDS as related to Autoimmune Diseases