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Rheumatoid arthritis is an autoimmune condition, which means it's caused by the immune system attacking healthy body tissue. However, it's not yet known what triggers this.

Your immune system normally makes antibodies that attack bacteria and viruses, helping to fight infection.

If you have rheumatoid arthritis, your immune system mistakenly sends antibodies to the lining of your joints, where they attack the tissue surrounding the joint.

This causes the thin layer of cells (synovium) covering your joints to become sore and inflamed, releasing chemicals that damage nearby:

• bones
• cartilage – the stretchy connective tissue between bones
• tendons – the tissue that connects bone to muscle
• ligaments – the tissue that connects bone and cartilage

If rheumatoid arthritis is not treated, these chemicals gradually cause the joint to lose its shape and alignment. Eventually, it can destroy the joint completely.

Various theories of why the immune system attacks the joints have been suggested, such as an infection being a trigger, but none of these theories has been proven.

There are several things that may increase your risk of developing rheumatoid arthritis, including:

• your genes – there's some evidence that rheumatoid arthritis can run in families, although the risk of inheriting it is thought to be low as genes are only thought to play a small role in the condition
• hormones – rheumatoid arthritis is more common in women than men, which may be because of the effects of the hormone oestrogen, although this link has not been proven
• smoking – some evidence suggests that people who smoke have an increased risk of developing rheumatoid arthritis

Further information

• National Rheumatoid Arthritis Society: possible causes and risk factors