Pediatric mixed connective tissue disease (MCTD) is a rare disease in children that has symptoms of arthritis along with 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 primarily works to protect the body from infections. However, in autoimmune diseases, immune cells attack the body's own tissues, resulting in inflammation and tissue damage.