An autoimmune disease is a condition in which the immune system, the body's natural defense system, starts to work abnormally.
Antibodies formed within the body begin to attack the body's own tissues.
Under normal circumstances, the immune system attacks only those things that are foreign to the body, such as bacteria. These immune system diseases can be life-threatening or associated with chronic systems. Any part of the organ system of the body may be attacked by these antibodies, including the blood, connective tissue, thyroid and nervous system. Immune responses normally protect from infections caused by bacteria, fungi, parasites and viruses and from developing cancer. The immune system provides antibodies that combine specifically with antigens that are foreign to the body.