Two protein molecules that fit together as lock and key seem to promote the abnormal formation of blood vessels in joints affected by rheumatoid arthritis, according to researchers who found that the substances are present at higher levels in the joints of patients affected by the disease. Rheumatoid arthritis is a chronic autoimmune inflammatory disease in which the body's own defenses attack the tissues lining the joints, causing painful swelling and bone erosion that can ultimately lead to joint deformities.