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Rheumatologists specialize in the diagnosis and treatment of certain autoimmune diseases. The purpose of the immune system is to fight off harmful substances in the body, such as infections. In order to do this, the immune system must be able to tell the difference between normal healthy parts of the body (self) and harmful intruders (non-self.), just as police officers try to distinguish "good guys" from "bad guys."

The immune system has many complex mechanisms for achieving this. Every once in a while, something goes wrong. If the immune system incorrectly labels a normal healthy part of the body as being harmful, it might attack the healthy part just as if it were a harmful bacteria. This is called autoimmunity. Autoimmune diseases are also commonly referred to as connective tissue diseases, because the connective tissue in the body is often the target.

The part of the body which is being attacked determines the type of autoimmune disease a person might have. For example, in rheumatoid arthritis the joints are attacked. In Lupus, the kidneys and skin can be attacked. In dermatomyositis, the skin and muscles can be attacked.