Skip to content

Food allergies

A food allergy is an abnormal, exaggerated reaction of the immune system to certain foods.

When someone has a food allergy, their immune system wrongly sees the food as hostile and the body's defence mechanism springs into action. This produces a range of symptoms, which can vary from mild itching to severe breathing difficulties or even shock. These symptoms usually happen immediately after eating the food.  

An allergen is any normally harmless substance that causes an immediate allergic reaction in a susceptible person. Food allergens are almost always proteins, although other food ingredients, such as some additives, are known to cause allergic reactions.


Sometimes a person can suffer an allergic reaction after eating a protein that is is similar to another protein that is an allergen. For instance,

  • A latex allergy can be associated with a number of food allergies including banana, avocado, chestnut, apple, carrot, celery, papaya, kiwi, potato, tomato and melons.
  • Allergy to tree pollen can also be associated with allergies to fruit.
  • People who are allergic to birch pollen are also allergic to apples, almonds, peaches, kiwi, carrots, celery, peppers and hazelnut.
  • Cross-reactivity with melons, tomatoes and oranges has been recorded in people who are allergic to grass pollen.

What happens in an allergic reaction?

Essentially, when the immune system reacts to a food ingredient during an allergic reaction, it triggers the release of chemicals such as histamine from cells in the body. This causes some or all of the following symptoms:

  • Itching or swelling in the mouth and throat
  • Hives anywhere on the body
  • Runny nose and eyes
  • Reddening of the skin
  • Feeling sick
  • Diarrhoea and/or vomiting

If the reaction is severe, other symptoms can occur including:

  • A sudden feeling of weakness (caused by a drop in blood pressure)
  • Breathing problems (your throat might start to swell up or close)

This is an anaphylactic reaction, also known as anaphylactic shock, and is life threatening. It requires immediate treatment by adrenaline injection followed by expert medical assistance. Usually the symptoms happen within seconds or minutes of being exposed to the food but the reaction can be delayed for several hours.

What foods cause an allergic reaction?

Although peanut and nut allergies are probably best known, any food can cause an allergic response in a susceptible person. Allergies to over 180 foods have been documented worldwide. Most of these are very rare and some are associated with particular populations or regions of the world.

Cod fish allergy is common in Scandinavia, as is rice allergy in China and celery allergy in France. These allergies are less common on the island of Ireland where the more frequently encountered allergies include those to peanuts, tree nuts, egg, crustaceans, milk and wheat.

How is a food allergy diagnosed?

  • A physical check-up will rule out other more obvious medical problems in the first instance.
  • A concise dietary history including a description of the symptoms and the foods suspected of triggering these symptoms.
  • A history of atopic disease in the family including any allergies to foods or other materials, any food intolerances including coeliac disease and incidences of asthma, hay fever, eczema, etc.
  • The patient may be instructed to keep a food diary of their eating habits, symptoms and medications to help pinpoint the problem.
  • An elimination diet may be used to link the symptoms with the food or foods causing them. This may not be advisable if the reaction has been severe.

If a food allergy is suspect after these investigations, more diagnostic methods are employed:

  • Skin prick tests are used to determine the reaction to a range of foods or to see if the problem could have been caused by other common allergens such as dust, cat hair or pollen. It can also give an indication of how strong the reaction is.
  • Blood tests are used to determine the strength of the immune system response to an allergen. This usually involves a radioallergosorbent or ‘RAST’ test in which the level of IgE antibodies – the kind specifically associated with a food allergic reaction – are measured.
  • Allergen provocation tests are generally regarded as being diagnostically the most definitive. The ‘double-blind placebo controlled food challenge’ (DBPCFC) is used to administer the suspect allergenic food to the patient orally under clinical supervision. The DBPCFC can be risky for people who may have a severe food allergy and in this case is only carried out in a hospital setting with full resuscitation equipment.

What is oral allergy syndrome?

In some cases allergic reactions can be confined to the mouth, lips, tongue or throat area. Here, the symptoms which are generally tingling and localised swelling usually occur within one hour of eating the offending food. Oral allergy syndrome is normally linked to the consumption of fresh fruit and vegetables. Patients often complain that something is stuck in their throat but progression to a severe reaction is unlikely. People with oral allergy syndrome caused by fruit and vegetables often have an associated allergy to certain pollens and may get hay fever when these pollens are in season.

What about exercise induced food allergy?

This is a rare condition where someone can have a severe allergic reaction within a couple of hours of eating a particular food, if they take vigorous exercise. People who are sensitive in this way may normally be able to eat the food with only a mild reaction or no reaction at all.

How can I find out if a food product is being withdrawn or recalled for food allergy reasons?

When allergy labelling is incorrect or inadequate or if there is another reason which puts food allergy sufferers at risk, the food product has to be withdrawn or recalled to protect consumers who can stay informed about the latest withdrawals or recalls by signing up for food allergy alerts by email or SMS text message.

These alerts are issued regarding the possible risk to food hypersensitive consumers from a particular food. In the Republic of Ireland, consumers can subscribe to the Food Safety Authority of Ireland (FSAI) allergen alerts.

Related pages

Safefood Logo

Sign up for our latest healthy eating and recipe updates.

Safefood logo

The site content is redirecting to the NI version.