Diseases of joints, muscles, bone, cartilage, and tendons are called connective tissue disorders because of the large amount of connective tissue contained in these structures. However, many connective tissue diseases are actually autoimmune diseases because they involve an immune response in which the connective tissue triggers the immune system to fight against its own tissues and produce abnormal antibodies that attach to these tissues (autoantibodies). The immune response is characterized by inflammation, which under normal circumstances represents a process of tissue repair, and the inflammation resolves on its own when the repair is complete. However, the inflammation in autoimmune diseases is a chronic inflammation that often leads to damage of normal tissues, as in rheumatoid arthritis where chronic inflammation causes destruction of articular cartilage. Probably because inflammation is triggered by antibodies circulating in the blood, in rheumatoid arthritis and many other autoimmune diseases, inflammation can spread to multiple joints. The connective tissues within and around the joints and in other parts of the body may become inflamed. Muscles, the pericardium around the heart, the outer membranes of the lungs (pleura), and even the meninges on the surface of the brain may be attacked by inflammation. The type and severity of symptoms depend on the organ(s) involved.