“If you’ve been healthy and suddenly you feel fatigue or joint stiffness, don’t downplay that,” she says. “Telling your doctor helps him or her to look closer at your symptoms and run tests to either identify or rule out autoimmune disease.” Researchers don’t know what causes autoimmune disease, but several theories point to an overactive immune system attacking the body after an infection or injury. The blood cells in the body's immune system help protect against harmful substances. Examples include bacteria, viruses, toxins, cancer cells, and blood and tissue from outside the body. These substances contain antigens. The immune system produces antibodies against these antigens that enable it to destroy these harmful substances.
When you have an autoimmune disorder, your immune system does not distinguish between healthy tissue and potentially harmful antigens. As a result, the body sets off a reaction that destroys normal tissues. The exact cause of autoimmune disorders is unknown. One theory is that some microorganisms (such as bacteria or viruses) or drugs may trigger changes that confuse the immune system. This may happen more often in people who have genes that make them more prone to autoimmune disorders