What is food allergy?

Some people react with what doctors call a hypersensitivity reaction when eating certain foods or additives. Typically it is commonly-consumed foods such as peanuts or eggs that cause a reaction.

For an adverse reaction to be called a food hypersensitivity, the symptoms that occur must be of a certain kind, specifically:

  • Appear when a person eats the problem food.
  • Disappear or diminish when a person avoids the problem food.
  • Reappear when the person reintroduces the problem food.

Food hypersensitivity is divided into two types, food allergy and non-allergic food hypersensitivity.

Food allergy
  • IgE-mediated food allergy, e.g.
    • Milk, egg, peanut, etc
    • Pollen-related
    • Latex-related
  • Non-IgE-mediated food allergy, e.g.
    • Coeliac disease (gluten)
Non-allergic food hypersensitivity (food intolerance)


  • Lactose (milk) intolerance
  • Hypersensitivity toward sulphites

The immune system and food allergy

People with food allergies have an allergic reaction when they come in contact with certain foods.

This happens because their immune system overreacts to the proteins in that food, 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 which can 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 Immunoglobulin E (IgE).

We produce IgE to fight infections caused by parasites, like worms or those that cause malaria.

The immune systems of people with allergy make IgE by mistake to harmless things like pollen or dust mites, giving rise to hay fever and asthma, and to some foods.

Allergens in food

Food allergens (the molecules in food responsible for an allergic reaction) are usually proteins. Typically, there are 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 antibodies that recognise 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.

How reactions occur

The IgE circulates in the blood and some of it attaches to the surface of specialised 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 sensitised to the food and primed to produce an allergic reaction.

On any following occasion when the person eats the same food, the allergens interact with the IgE on the surface of the mast cells. In response, the activated mast cells rapidly release chemicals such as histamine.

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.

Related allergies

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.

Further reading