Autoimmune diseases are disorders in which the immune system attacks the body's own cells. They are among the most prevalent classes of public health problems.
For example, the cause of autoimmune type 1 diabetes (T1D) is the destruction of the insulin-producing cells of the pancreas by the immune system. The loss of insulin results in high glucose levels in blood that cause the long-term complications of this disease.
When T lymphocytes destroy nerve tissues, the resulting damage causes multiple sclerosis.
The basis of autoimmune disease is the immune system's loss of "tolerance" to the body's own tissues. That is, the immune system fails to recognize that the person's own tissues are not foreign and should not be attacked.
Cells called T lymphocytes normally help fight against infection and other material foreign to the body ("non-self").
However, certain rogue cells called "autoreactive T lymphocytes" respond to the body's own tissues and organs as if they were foreign; that is, they lose "tolerance" to the body's own cells and tissues.
Usually the body destroys such cells soon after the immune system makes them, but when such autoreactive T lymphocytes evade elimination and mature, they can cause serious diseases.
Physicians usually treat autoimmune diseases with an immunosuppressive drug that decreases the activity of the immune system so it does not attack the person's own tissues or transplanted organs or tissues (e.g. islet transplants as treatment of T1D). The disadvantage of immunosuppressive drugs is that they not only suppress the attack on the patient's own cells (or transplanted cells) but also hinder the ability of the immune system to fight infectious diseases.