Systemic lupus erythematosus (S.L.E.), commonly called lupus, is a chronic autoimmune disorder that can affect virtually any organ of the body. In lupus, the body's immune system, which normally functions to protect against foreign invaders, becomes hyperactive, forming antibodies that attack normal tissues and organs, including the skin, joints, kidneys, brain, heart, lungs, and blood. Lupus is characterized by periods of illness, called flares, and periods of wellness, or remission. Because its symptoms come and go and mimic those of other diseases, lupus is difficult to diagnose. There is no single laboratory test that can definitively prove that a person has this complex illness.