This medication is used to treat rheumatoid arthritis, a condition in which the body's own defense system (immune system) attacks healthy tissue. This leads to swelling in the joints, which causes pain and makes it harder to move. Abatacept works by weakening your immune system. This effect helps to slow down joint damage and reduce joint pain and swelling so you can move better. This medication is also used to treat other types of arthritis (such as juvenile idiopathic arthritis, psoriatic arthritis).