Causes of reactive arthritis

It’s not known exactly what causes reactive arthritis, but it is thought to be the result of the immune system reacting to a previous infection.

The immune system

Your immune system is your body’s defence against illness and infection. When it senses the presence of an infectious agent, such as a virus or bacteria, it will send antibodies and cells to fight the infection.

These antibodies then release chemicals that cause nearby tissues to swell and become filled with fluid. This process is known as inflammation.

Inflammation makes it harder for the infection to spread and can allow the body to kill the bacteria or virus responsible for the infection.

In cases of reactive arthritis, something goes wrong with the immune system and it acts like there’s still an infection in the body, even though the previous infection has 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 cases occur after non-gonococcal urethritis
• infections of the bowel and digestive system (gastrointestinal infections) – usually caused by two types of bacteria called campylobacter and salmonella, both of which can cause food poisoning

Genetic factors

Research has shown that people with a specific type of gene known as HLA-B27 have an increased chance of developing reactive arthritis, as well as other autoimmune conditions such as ankylosing spondylitis, a type of arthritis that affects the spine.

In the UK, it’s estimated that around one in 10 people have the HLA-B27 gene. Three out of four cases of reactive arthritis develop in people with the HLA-B27 gene. They tend to have more severe symptoms, with a greater risk of their symptoms reoccurring.

One study found that people with the HLA-B27 gene had around a one in 50 chance of developing reactive arthritis after contracting a stomach or digestive infection. It’s unclear whether there’s a similar risk after contracting a STI.

Exactly how HLA-B27 contributes to the development of reactive arthritis is unclear, although research is ongoing.