What is the cause of rhemetoid arthritis?

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:
cartilage – the stretchy connective tissue between bones
tendons – the tissue that connects bone to muscle
ligaments – the tissue that connects bone and cartilage
If the condition isn’t 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 starts to attack the joints have been suggested, such as an infection or virus being a trigger, but none of these theories has been proven.