The small intestine is largely responsible for digesting and extracting nutrients from the food that we eat. Many thousands of different molecules cross from the intestines into the blood without causing damage.
The immune system, which identifies invading pathogens and attacks and destroys them, does not recognise food as harmful in the same way. Sometimes, though, things can go wrong.
When the immune system reacts to a molecule that should pass through without causing a problem, this can lead to an allergic response.
Allergies often lead to swelling and inflammation in the area of the body where the allergen comes into close contact with body tissues. For example hay-fever, which is an allergy to pollen, causes inflammation in the nose when the pollen is inhaled.
People with coeliac disease, which is also known as celiac disease or celiac sprue, have an allergic response to the protein gluten.
If gluten gets into their small intestine, the immune system becomes fully activated, causing local inflammation, soreness and pain. This sets off the typical symptoms of coeliac disease.