A person has a food allergy when they cannot tolerate one or more foods and their immune system is involved in creating the symptoms.
Our immune system protects our bodies from infections. We produce molecules, called antibodies, which recognise germs that cause infections. Our immune system makes a number of different types of antibody, which have different roles. The one that plays a role in an allergic reaction is called IgE. We produce IgE molecules to fight infections caused by parasites, like worms or those that cause malaria. We do not understand why, but the immune system of some people makes IgE by mistake to harmless things like pollen or dust mites, giving rise to hay fever and asthma, and to some foods, giving rise to food allergies.
Food allergens (the parts or molecules in food responsible for an allergic reaction) are usually proteins. There are generally several different kinds of allergen in each food. It is not yet clear what makes some proteins food allergens, and not others.
When a person eats a food, the food may trigger immune cells to produce large amounts of IgE that recognises that food. Sometimes the immune cells can be triggered to produce IgE when a person breathes in tiny parts of a food e.g. sunflower seeds when they are used to feed birds. The IgE circulates in the blood and some of it attaches to the surface of specialized inflammatory cells called mast cells. These cells occur in all body tissues but are especially common in areas of the body that are typical sites of allergic reactions. The person is then sensitized to the food and primed to produce an allergic reaction.
On any subsequent occasion when the person eats the same food, the food allergens interact with the specific IgE on the surface of the mast cells. In response, the activated mast cells rapidly release chemicals such as histamine. Depending upon the tissue in which they are released, these
chemicals will cause a person to have various symptoms of food allergy. It varies how much of the allergen that have to be present to cause symptoms. In some instances even very small amounts of an allergen may trigger severe symptoms. For some people it is enough to just breathe in a tiny part of the food to get an allergic reaction.
Some people have allergic reactions where IgE is not involved. Gluten hypersensitivity (coeliac disease) is an example of non-IgE-mediated food allergy.
Individuals with pollen or latex allergy often experience allergic symptoms when they eat certain fruits, vegetables or nuts. This “cross-reactivity” occurs because the body cannot distinguish between the allergens in pollen or latex and related proteins in food and may react to both.