Rheumatoid arthritis occurs when the immune system, the body’s defense against invading viruses and bacteria, starts to attack the membranes that surround the joints called the synovium. Immune system cells attack these tissues causing inflammation, swelling, pain, and stiffness. The disease process starts with swelling of the synovium causing pain, redness, and swelling which are characteristic of inflammation. Then, the cells of the synovium divide rapidly causing it to thicken and invade the synovial cavity that normally exists between bones at each joint. This thickening is called pannus or granulation tissue. Then, immune system cells start releasing enzymes that damage bone and cartilage. This causes severe pain and eventually disrupts the joint structure and destroys the smooth surfaces needed for joint motion.

While it is clear that an autoimmune reaction causes RA, it is not clear what causes that reaction to occur in certain people and not others. It is likely that several factors interact to cause the disease including genetics, lifestyle, and environmental factors such as exposure to certain viruses.