An auto immune diseas is

Explanation

The healthy human body produces lymphocytes (a type of WBCs) which produce antibodies. These antibodies attack and kill the invading pathogens such as bacteria, virus and other parasites. However, the antibodies have a natural ability to recognize the foreign cells and are selective in attacking them without causing any harm to the host tissues.
But, in some persons, the lymphocytes produce some antibodies called auto antibodies which run in the blood stream attack and damage the healthy tissues of the same individual. This may result in some disorder in the functioning of that organ or system. These are called auto immune disorders.
For example, rheumatoid arthritis, multiple sclerosis, psoriasis, vasculitis, etc.