Autoimmune diseases (also commonly called autoimmune disorders) are illnesses caused by a malfunctioning immune system.
A healthy immune system helps the body fight disease. It makes proteins called antibodies that identify and attack substances that may harm a person. Any substance that leads a person’s immune system to make these antibodies is called an antigen. Some antigens, such as viruses and certain bacteria, may threaten a person’s health. But sometimes, a person's immune system is hypersensitive and attacks antigens that are harmless to most other people.
In an autoimmune disease, the immune system misidentifies a substance that naturally occurs inside the body as a threat. It then makes antibodies to fight these substances (called autoantigens). In other words, the body attacks itself.