Mixed connective tissue disease (MCTD) is an autoimmune disease. Much like other autoimmune diseases, MCTD involves a malfunction in the immune system causing it to attack otherwise healthy cells. In the case of MCTD, the body's immune system wrongly attacks connective tissue that is completely healthy. MCTD is considered present when someone experiences symptoms in common with two or more of the other connective tissue diseases, including lupus, scleroderma, and polymyosis. MCTD also sometimes has symptoms in common with rheumatoid arthritis.