Food Allergy

Food Allergy occurs when your immune system overreacts to a food by mistakenly labelling it as a threat and triggering an immune response. The symptoms resulting from this immune response are often mild but they can also be very serious and trigger a life threatening reaction known as anaphylaxis

Symptoms of Food Allergy

The symptoms of food allergy may involve the skin, airways and digestive tract. The most common symptoms are:

  • red raised itchy rash or hives (urticaria)
  • itchy sensation inside the mouth, throat or ears
  • swelling of the lips, eyes, tongue or mouth (angioedema)
  • vomiting

In the most serious cases, a person has a severe allergic reaction (anaphylaxis), which can be life threatening. The symptoms of anaphylaxis include:

  • breathing difficulties
  • trouble swallowing
  • feeling faint or lightheaded

Common Food Allergies

An allergic reaction can be caused by almost any food but certain foods are responsible for the majority of food allergies.

The foods that most commonly cause an allergic reaction are:

  • milk
  • eggs
  • nuts
  • fish and shellfish
  • certain fruit and vegetables

Causes of Food Allergy

Although it is not well understood as to why people develop allergies to food, it is clear that they often have other allergic conditions such as asthmahay fever and eczema.

Most children that have a food allergy will have had eczema during infancy. When a child's eczema begins at an earlier age and is more severe they are at increased risk of developing food allergy.

Introducing nuts to your child's diet early (from age 7 months) greatly reduces the risk of developing a nut allergy. 

Types of Food allergies

Food allergies are divided into 3 types:

  • IgE mediated: This is the most common type and is triggered by the immune system producing an antibody called immunoglobulin E (IgE) in response to a specific food. Symptoms occur within 30 minutes of eating. Anaphylaxis is most commonly associated with this type of allergy.
  • Non IgE mediated: These allergic reactions are caused by other components of the immune system rather than IgE. This type of allergy is more difficult to diagnose as reactions do not occur immediately after the ingestion of the food. The symptoms are often gastrointestinal such as vomiting, bloating and diarrhoea. 
  • Mixed IgE and non-IgE mediated: Some food allergies occur through both of the above mechanisms.

Food Allergy and Eczema

People who have eczema with elevated IgE levels are more likely to have food allergies and airborne allergies. Current research indicates that skin barrier defects found in eczema alter immune function and can induce food allergy secondarily. Primary sensitisation is more likely to occur when new allergens are introduced through inflamed eczematous skin rather than the gastrointestinal tract. It is therefore important that parents avoid introducing new foods, especially nuts and shellfish, during any acute exacerbation of eczema in infancy.


  • Avoidance: The best way to prevent an allergic reaction to food is to identify the culprit and avoid it. Ensure that your speak with your GP first before cutting out any foods from your or your child's diet.
  • Mild - Moderate Allergic Reactions: Antihistamines can help relieve mild symptoms.
  • Severe Allergic Reactions / Anaphylaxis: Adrenaline is the primary treatment used in these cases. e.g. Epipen

Food Allergy Testing

If you think you or your child has a food allergy make sure you have this confirmed by allergy testing. It is important that the results are interpreted by a specialist medical doctor.

Perform an allergy test is easy, the skill lies in interpreting the result in the context of the patient's symptoms. Sometimes there is no link between the suspected allergen and the symptoms. As the case study below explains, trying to be your own detective can be misleading and take you along the wrong path and away from recovery.

When to seek medical advice

If you think you or your child may have a food allergy, it's very important to ask for a professional diagnosis from your GP. If appropriate, they can then recommend a reputable specialist allergy testing clinic such as Allergy Ireland for expert interpretation.

Many parents mistakenly assume their child has a food allergy when their symptoms are actually caused by a completely different condition.

Commercial allergy testing kits are available, but using them isn't recommended. Many kits are based on unsound scientific principles. Even if they are reliable, you should have the results looked at by a health professional.

Food labelling

Become a LABEL DETECTIVE when you buy food for your family. Labels and ingredient listings can change without warning. There is a risk of incorrect labelling of products with imported goods. Labelling mistakes can occur during translation.

  • Read all product labels every time you purchase a product. 
  • Check labelling on both outer and inner packages as discrepancies on food ingredient labelling can occur between the outer and inner packages in multi-packs.

Further Information for Health Professionals

Allergy Pathophysiology

There are two phases which are paramount to the development of an allergy. Phase one occurs when an atopic individual is first exposed to the allergen. The allergen is taken up by antigen presenting cells particularly dendritic cells (DC) and is processed into peptide fragments. The DC will move through the lymphatics towards the lymph node where it will present this peptide fragment to a naïve T cell. The naïve T cell becomes activated to express cytokines particularly IL-4 which drives the differentiation of these cells to Th2 helper cells.

An environment rich in cytokines IL-4 and IL-13 is created and is responsible for inducing IgE production from B cells. Additionally, IL-5 is responsible for eosinophil recruitment and activation. The cytokine profile is vital as it determines a Th2 immune response.

In the meantime, T cell dependent activation of B cells stimulates further cytokine production particularly IL-4 and promotes irreversible immunoglobulin class switching to allergen specific IgE antibodies. Allergen specific IgE will attach to mast cells and basophils. This is referred to as primary sensitisation. In addition, memory B cells are generated and a small number of memory T cells remain.

Illustration below from Bousquet et al. Nature Reviews Disease Primers, 2020

Phase two occurs on subsequent exposure to this allergen. The allergen binds to the sensitised mast cells triggering degranulation of the mast cell releasing pre-stored and newly synthesised inflammatory mediators such as histamine, leukotrienes and prostaglandins. These contribute to vascular permeability, eosinophil infiltration and increased mucus production. 

Pollen Food Allergy Syndrome (PFAS) 

PFAS is also known as Oral Allergy Syndrome and it is caused by cross reactivity of food proteins with pollens such as grass, birch and ragweed. The immune system of those affected misidentifies pollen with proteins of similar appearance found in certain foods and directs an immune response against them. The symptoms include itch, tingling and swelling along the inside of the mouth, lips, tongue and throat after ingestion of certain foods. The symptoms usually remain localised to the mouth and resolve quickly without treatment. In a small percentage of patients the symptoms may extend further than the mouth and this subset may be at risk of Anaphylaxis

Pollen Food Allergy Syndrome is most often seen in older children and adolescents who have been eating the same foods for years without any issues. Young children under the age of 3 are not affected as they are too young to have developed hay fever as yet.

Common Triggers

  • Birch tree pollen allergy: curry spices, raw tomato, raw carrot, celery, apple, pear, plum, peach, kiwi, cherry, hazelnut or almond.
  • Grass pollen allergy: oats, rye, wheat, raw tomato, celery, oranges, melons, peaches, kiwi.
  • Ragweed pollen allergy: banana, cucumber, melon, zucchini/courgette, sunflower seeds, chamomile tea or Echinacea. 

Management usually involves avoiding the uncooked form of the food. Cooking the food alters the shape of the misidentified protein thereby avoiding an immune response. If symptoms are still triggered with cooked food then the food must be avoided. Where high risk foods are involved or more profound symptoms are triggered then it may be necessary to carry an adrenaline pen.