The cause of rheumatoid arthritis (RA) is unknown. However, RA involves an attack on the body by its own immune cells (auto-immune disease). RA is classified as an autoimmune disease in which certain cells of the immune system malfunction and attack healthy joints. Different cases may have different causes. Infectious, genetic, and hormonal factors may play a role.

Genes may play some as yet unknown role. It also is possible that a change in the body, such as an infection or hormonal shift, can trigger its development.

A combination of factors — genetic, environmental and hormonal — probably play a role in the onset of the disease. Those with a genetic susceptibility may develop RA when it is triggered by an environmental agent, perhaps a virus or bacterium, though no pathogenic agent has yet been identified. Hormones also appear to have a role.