Illustration of the antigen binding fragment (Fab) of a rheumatoid factor autoantibody bound to the tail, or Fc (fragment crystallisable), region of its target immunoglobulin G antibody. Rheumatoid arthritis is an autoimmune disease where the joints become inflamed due to attack by the body's own immune system. Antibodies are proteins of the immune system that bind to specific molecules, or antigens. Autoimmune diseases occur when an antibody wrongly targets a molecule on a healthy cell of the body's own tissues.