Rheumatoid arthritis (RA) is a chronic, systemic inflammatory disorder that primarily affects joints.It may result in deformed and painful joints, which can lead to loss of function. The disease may also have signs and symptoms in organs other than joints.

The cause of RA is not completely understood. The process involves an inflammatory response of the capsule around the joints (synovium) secondary to swelling (turgescence) of synovial cells, excess synovial fluid, and the development of fibrous tissue (pannus) in the synovium. It also affects the underlying bone (focal erosions) and cartilage (thinning and destruction).

RA can also produce diffuse inflammation in the lungs, the membrane around the heart (pericardium), the membranes of the lung (pleurae), and whites of the eye (sclerae); and can also produce nodular lesions, most common in subcutaneous tissue. It is a clinical diagnosis made primarily on the basis of symptoms and physical examination. X-rays, laboratory testing, and synovial fluid analysis might help support a diagnosis or exclude other diseases with similar symptoms.