Pediatric mixed connective tissue disease (MCTD) is a rare disease in children. It has symptoms of arthritis and features of scleroderma, dermatomyositis and lupus. It is characterized by lab tests showing the presence of specific antibodies to nuclear proteins (specifically, RNP).

MCTD is a type of autoimmune disease. The immune system works to protect the body from infections. But, in autoimmune diseases, immune cells attack the body's own tissues. This results in inflammation (swelling) and tissue damage.