Rheumatoid arthritis (RA) is an autoimmune disease in which the body’s immune system — which normally protects the body by attacking foreign substances like bacteria and viruses — mistakenly attacks the joints.

In the early stages, people with RA may not initially notice redness or swelling in the joints, but they may experience tenderness and pain.

Rheumatoid arthritis most commonly affects the joints of the hands, feet, wrists, elbows, knees, and ankles. The joint involvement is usually symmetrical. That means if one knee or hand is affected, usually the other one is too.

Because RA can also affect other body systems, such as the cardiovascular or respiratory systems, it is called a systemic disease. “Systemic” means affecting the entire body.