Rheumatoid arthritis occurs when white blood cells â€” whose usual job is to attack unwanted invaders, such as bacteria and viruses â€” move from your bloodstream into the membranes that surround your joints (synovium).
The blood cells appear to play a role in causing the synovium to become inflamed. The inflammation causes the release of proteins that, over months or years, cause the synovium to thicken. The proteins can also damage the cartilage, bone, tendons and ligaments near your joint. Gradually, the joint loses its shape and alignment. Eventually, it may be destroyed.
Doctors don't know what causes this process that leads to rheumatoid arthritis.
It's likely that rheumatoid arthritis occurs as a result of a complex combination of factors, including your genes, your lifestyle choices, such as smoking, and things in your environment, such as viruses.