Lupus is an autoimmune disease, a disease that occurs when the immune system, the body’s natural system for defending itself against pathogens, starts attacking normal body tissues. Normally the body’s immune system makes proteins called antibodies to protect the body against viruses, bacteria, and other foreign materials. In lupus, the immune system makes antibodies directed against normal “self” molecules referred to as autoantibodies. The most common type of autoantibody in people with lupus is called an antinuclear antibody because it is directed against the nucleus, the control center found in most cells.

Autoantibodies lead to inflammation, which can occur in any of several parts of the body. Inflammation is considered the primary feature of lupus and wherever in the body it occurs, symptoms appear. Symptoms are often worse in body regions where there is greater inflammation.

Lupus is a complex disease and researchers do not yet know why autoantibodies develop in the first place, or why they tend to attack certain tissues in some people who have lupus but not in others. Current thinking holds that lupus, and the autoantibodies and inflammation that cause it, probably result from a combination of factors both genetic and environmental. Some of the environmental factors under study include sunlight, stress, drugs, and viruses. The fact that so many more women than men get the disease also suggests that hormones may play a role in the disease.