What exactly is an autoimmune disease?

Difficult: The tissues in the body respond to an antigen that acts like a foreign body causing symptoms.

Autoimmune diseases : In auto immune disease the enemy is from with in . As name implies "auto" is self and "immune " is immune system . One 's own immune system for unknown reason turns against self and destroys or damages tissues or cells . List of auto immune disease are many and growing . End result is destruction of of tissues such as thyroid , pancreas or cells such as white cells , platelets etc.