Doctors don't know exactly what causes rheumatoid arthritis (RA). Some experts believe the immune system becomes "confused" after infection with a bacteria or virus and starts to attack your joints. This battle can spread to other areas of the body. Scientists think two of the body's chemicals that are related to inflammation, tumor necrosis factor (TNF) and interleukin-1, trigger other parts of the immune system in rheumatoid arthritis. Medicines that block TNF, interleukin-1, and interleukin-6 can improve RA symptoms and prevent joint damage.