OA is a degenerative disease that affects the joints, especially larger, weight-bearing joints (knees, hips, and spine). In the United States, 27 million people have OA. The cartilage in the joints breaks down, causing swelling, pain, and difficulty moving. RA is a systemic autoimmune disease, in which the immune system malfunctions and attacks the body’s own tissues. RA inflammation causes the cartilage in the joints to be eaten away, which also leads to swelling, pain, and difficulty moving. It can also affect other parts of the body, such as your organs and vascular system. About 1.3 million Americans have RA.