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. An autoimmune disorder may result in: The destruction of body tissue, Abnormal growth of an organ, Changes in organ function. An autoimmune disorder may affect one or more organ or tissue types. Areas often affected by autoimmune disorders include: Blood vessels, Connective tissues, Endocrine glands such as the thyroid or pancreas, Joints, Muscles, Red blood cells, Skin. A healthy immune system defends the body against disease and infection. But if the immune system malfunctions, it mistakenly attacks healthy cells, tissues, and organs. Called autoimmune disease, these attacks can affect any part of the body, weakening bodily function and even turning life-threatening.