Causes of reactive arthritis

It's not known exactly what causes reactive arthritis, but it's thought to be the result of the immune system reacting to an infection.

The immune system

Your immune system is your body's defence against illness and infection. When it senses a virus or bacteria, itsends antibodies and cells to fight the infection. This causes tissues to swell, known as inflammation, which makesit harder for theinfection to spread.

In cases of reactive arthritis, something goes wrong with the immune system and it causes inflammation in parts of the body that were not infected, often after the infection has already passed.

Infection

The two most common types of infection linked to reactive arthritis are:

• sexually transmitted infections (STIs) such as chlamydia or gonorrhoea , although many casesoccur after non-gonococcal urethritis
• infections of thebowel and digestive system (gastrointestinal infections) usuallycaused by either campylobacter or salmonella bacteria, both of which can cause food poisoning

Genetic factors

Research has shown that people with a specific gene known as HLA-B27 have a significantlyincreased chance of developing reactive arthritis, as well as related conditions such as ankylosing spondylitis (a type of arthritis that affects the spine) .

In the UK, it's estimated thataround1 in every 10 people have the HLA-B27 gene. Around3out of every4 cases of reactive arthritis develop in people with the gene.

People withthe HLA-B27 gene also tend to have more severe and longer-lasting symptoms, with a greater risk of their symptoms recurring.

Exactly how the gene contributes to the development of reactive arthritis is unclear.