Causes of rheumatoid arthritis
The exact cause of rheumatoid arthritis is not clear. It is known that it is an autoimmune disease – that is, a disease caused when the immune system malfunctions and attacks healthy tissues in the body – but exactly what triggers this malfunction has not been identified. Most likely it is a combination of several factors.Infection
Infections trigger the body’s immune system to produce antibodies (specialised proteins that bind to infectious agents in the body and help to destroy them). Antibodies are individually shaped to bind to a specific target, called an antigen, so that a particular antibody should react only with the particular virus or bacteria it is intended to destroy – like a key that only fits into only one lock. In rheumatoid arthritis, it is thought that antibodies designed to eliminate a virus or bacteria may ‘cross react’ with a protein found on the membranes’ lining joints – like a key that can open a different lock due to its similarity in shape. The binding of an antibody to an antigen triggers an inflammatory response and this could be what causes rheumatoid arthritis.
Infections that have been proposed as potential triggers for rheumatoid arthritis include Epstein Barr virus (the virus that causes glandular fever), certain types of herpes virus, the rubella virus, and the bacteria Mycoplasma (which can cause lung infections), Proteus (which can cause urine infections) and Mycobacteria (which can cause TB).
Hormonal factors
Rheumatoid arthritis often develops in women at around the time of the menopause and it has been suggested that this may be due to a fall in the level of the female hormone oestrogen. Oestrogen reduces the production of inflammatory chemicals in the body, helping to keep inflammation in check. Oestrogen deficiency may therefore be associated with the development of inflammation.
This theory is supported by the fact that the oral contraceptive pill, which contains oestrogen, appears to protect against the development of rheumatoid arthritis; and that the symptoms of rheumatoid arthritis often improve during pregnancy, when oestrogen levels are high. However, the effects of hormone replacement therapy on rheumatoid arthritis are unclear.
Genetic factors
No one gene causes rheumatoid arthritis but certain genes have been identified that can make a person more susceptible to it. It is thought that these genes lower the threshold that must be reached for an inflammatory reaction to be triggered.
