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Food allergies explained

A food allergy is an abnormal immune system reaction to a food.

What is a food allergy?

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 symptoms which range from mild itching to severe breathing difficulties or even shock. The symptoms usually happen immediately after eating the food.

What are the symptoms of an allergic reaction to food?

During an allergic reaction to food, the immune system reacts to a food ingredient and triggers the release of chemicals from cells in the body like histamine which cause runny nose or sneezing. An allergic reaction causes some or all of these symptoms.

  • Itching or swelling in the mouth and throat
  • Hives
  • 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 with adrenaline followed by expert medical assistance. Usually, the symptoms happen within seconds or minutes of being exposed to the food, but the allergic reaction can also be delayed for several hours.

What foods can cause an allergic reaction?

On the island of Ireland, the most common food allergies are those to peanuts, tree nuts, egg, crustaceans and milk. Allergies to over 180 foods have been documented worldwide. Most of these are rare and some are more common among certain populations or regions. Cod fish allergy is common in Scandinavia, as is rice allergy in China and celery allergy in France.

How is a food allergy diagnosed?

If you think you may have a food allergy, then talk to your doctor or a registered dietitian as soon as possible. As well as investigating the symptoms of a potential food allergy, your doctor will also check your medical history for other food allergies or allergic conditions and whether there is a history of these in your family. If your GP suspects a food allergy, they may refer you to an allergy specialist who can carry out further tests.

Don’t self-diagnose

Never attempt any allergen test by yourself as many of the symptoms associated with a food allergy or intolerance are common to other illnesses and you may misdiagnose.

What is cross-reactivity?

Sometimes a food allergen can be very similar to another allergen in a food or other material. If a person is allergic to one type of food, then they might also react to other types of food as well. For instance,

  • People who are allergic to birch pollen are sometimes 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 is oral allergy syndrome?

In some cases, allergic reactions are only in the mouth, lips, tongue or throat area. This is called oral allergy syndrome and the symptoms include tingling and localised swelling which happens usually within one hour of eating the food (which is usually a fruit or vegetable.

An oral allergy syndrome is unlikely to develop into a severe allergic reaction. It often feels like something is stuck in your throat. 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?

An exercise induced food allergy 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 be able to eat the food with only a mild reaction or no reaction at all if they don’t exercise heavily afterwards.

How can I protect myself when I have been diagnosed with a food allergy?

The best way is become very familiar with ingredient labelling (which includes allergens) on food products. You also need to be extra careful when eating out. Allergenic ingredients should be provided in writing or verbally by the restaurant, catering or take out service. However, never be complacent and always ask the staff about allergens and the possibility for cross contamination.

Food withdrawals and recalls can occur if an allergy label is incorrect, inadequate or the product puts food allergy suffers at risk in some way.

In Ireland, you can sign up to the Food Safety Authority of Ireland (FSAI) allergen alerts to stay informed of withdrawals or recalls.

In the UK, you can subscribe to the Food Standards Agency’s ‘News and alerts’ resource which includes alerts concerning food allergens.  

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