The most common types of arthritis are rheumatoid arthritis (RA) and osteoarthritis (OA). RA is a long-term autoimmune disorder in which the body attacks itself. If you have RA, your body interprets the soft lining around your joints as a threat, similar to a virus or bacteria, and attacks it. This attack causes fluid to accumulate within your joints. In addition to swelling, this fluid buildup causes pain, stiffness, and inflammation around your joints.