Coeliac disease is caused by an abnormal immune system reaction to the protein gluten, which is found in foods such as bread, pasta, cereals and biscuits.

It's an autoimmune condition, where the immune system mistakes healthy cells and substances for harmful ones and produces antibodies against them (antibodies usually fight off bacteria and viruses).

In the case of coeliac disease, your immune system mistakes one of the substances that makes up gluten, called gliadin, as a threat to the body. The antibodies that are produced cause the surface of your intestine to become inflamed (red and swollen).

The surface of the intestine is usually covered with millions of tiny tube-shaped growths called villi. Villi increase the surface area of your gut and help it to digest food more effectively.

However, in coeliac disease, the damage and inflammation to the lining of the gut flattens the villi, reducing their ability to help with digestion.

As a result, your intestine is not able to digest the nutrients from your food, which causes the symptoms of coeliac disease.

Some people with coeliac disease may find that eating oats can trigger symptoms. This is because some oats may be contaminated by other grains during production.

Oats also contain a protein called avenin, which is similar to gluten. Most people with coeliac disease can safely eat avenin. However, there's some evidence to suggest a very small number of people may still be sensitive to products that are gluten-free and do not contain contaminated oats.

It's not known why people develop coeliac disease. It's also not clear why some people have mild symptoms while others have severe symptoms.

However, several factors are known to increase your risk of developing coeliac disease.

Family history

Coeliac disease often runs in families. If you have a close relative with the condition, such as a parent, brother or sister, your chance of also getting it is increased.

This risk is approximately 10% for those with a family history. If you have an identical twin with coeliac disease, there's around a 75% chance you'll also develop the condition.

Research shows that coeliac disease is strongly associated with abnormal changes to the instructions that control cell activity (genetic mutations). These mutations affect the HLA-DQ genes, which are a group of genes responsible for the development of the immune system and may be passed down through a family.

However, mutations in HLA-DQ genes are very common. This suggests that something else, such as environmental factors, must trigger coeliac disease in certain people.

Environmental factors

You're more likely to develop coeliac disease if you had a digestive system infection (such as a rotavirus infection) during early childhood.

Most experts recommend waiting until your child is at least 6 months old before giving them food containing gluten.

There might also be an increased chance of babies developing coeliac disease if they're not being breastfed when gluten is introduced into their diet.

Find out more about your baby's first solid foods.

Other health conditions

Other health conditions can increase your risk of developing coeliac disease, including:

• type 1 diabetes
• thyroid conditions
• Down's syndrome 
• Turner syndrome

It's unclear whether these health conditions directly increase the risk of developing coeliac disease, or whether they and coeliac disease both have another, single underlying cause.