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Food allergy week #1 – what is food allergy?


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Brayden is a 3 year old boy who has been referred to your clinic due to concerns about food allergies. He has been having a lot of flare ups of his eczema. On only one occasion he developed some facial swelling and wheeze after ingesting peanuts. The GP was wondering if it could be related to specific foods.

This post is part of a series of posts on food allergy. The other post in series are: #2 – other types of food allergies; #3 diagnosing food allergy; #4 – management of food allergy in infants.


Is food allergy becoming more common?

There have been an increasing number of allergies in recent years, especially in young children.

It occurs in:

10% of infants 1 year and below, 4-8% of children under 5 years old, and up to 2% of the adult population in Australia

There is no clear explanation for the increase, but proposed ones include:

  • Hygiene hypothesis
  • Delayed introduction of allergenic foods
  • Methods of food processing
  • Development of allergy to food by skin exposure
  • Vitamin D deficiency in the first year of life
  • Medication to suppress gastric acid production

What exactly is food allergy?

The definition is:

an abnormal immune mediated reaction to ingested food, resulting in clinical symptoms

Food allergies can be classified by their immune mechanism:

  • IgE mediated (acute urticaria, oral allergy syndrome)
  • Non-IgE mediated (food protein-induced enteropathy syndrome)
  • Mixed IgE and non-IgE mediated (eosinophilic oesophagitis)
  • Cell mediated (allergic contact dermatitis)

What's the difference between food allergy and food intolerance?

Food intolerance does not involve the immune system and does not result from IgE mediated reactions nor does it cause anaphylaxis.

The exact mechanism by which some food intolerance occurs is not always clear.

There is no reliable skin or blood test to diagnose food intolerances, despite claims of an ability to do so using unproven/unorthodox testing methods (e.g. kinesiology, iridology, IgG food testing).

Non-immune mediated food reactions include:

  • Metabolic (lactose intolerance)
  • Pharmacologic (caffeine)
  • Toxic (scombroid fish toxin)
  • Other (sulphites)

How do mild-moderate reactions present?

These usually appear within 1-2 hours of ingesting the food and include:

  • Swelling of lips, face, eyes
  • Hives or welts
  • Tingling mouth
  • Abdominal pain, vomiting
  • Eczema or rashes

How do severe reactions present?

  • Difficult/noisy breathing
  • Swelling of tongue
  • Swelling/tightness in throat
  • Difficulty talking and/or hoarse voice
  • Wheeze or persistent cough
  • Persistent dizziness or collapse
  • Pale and floppy (young children)

Note: previous mild or moderate reactions do not rule out a risk of subsequent anaphylaxis!

What are the most common food allergies?

90% of food allergies in children involve peanuts, tree nuts, sesame, cow’s milk, soy, egg, shellfish, fish, or wheat.

However, ANY food including fruits and vegetables can cause anaphylaxis.

Allergic reactions can occur on first known exposure to food. True initial exposure and sensitisation to allergens may be unintentional and unknown.

The most common cause of fatal food anaphylaxis is due to peanut allergy.

Is there cross-reactivity to other foods?

Cow’s milk protein – 10% are also allergic to soy. This is particularly relevant when choosing an alternative infant formula or cow’s milk substitute.

Egg – 30% also have peanut or tree nut allergy.

In certain patients, allergy testing for co-reactive foods may provide useful information on co-existing allergy.

These are examples where the patient is clinically allergic to proteins in unrelated foods. However, more common is where there is clinical allergy to similar proteins present in related foods.

  • Cow’s milk: ~90% will be allergic to goat’s milk
  • Cashew: almost all will be allergic to pistachio
  • Walnut: most will be allergic to pecan
  • Fish: ~75% will be allergic to other fish
  • Prawn: most will be allergic to other crustaceans (e.g. crab, lobster)
  • Peanut: ~5% are allergic to another legume (e.g. soy)
  • Oral allergy syndrome: allergic to similar proteins in pollen and some fruit/vegetables

Are food allergies for life?

Peanut, treenut, sesame, shellfish, and fish allergies usually persist into adulthood.

Extensively heated egg or cow’s milk products (e.g. baked goods) may be tolerated in children who have shown clinical allergy to raw products. If tolerated, they can be continued regularly.

Few studies suggest, but there is inconclusive evidence, as to whether regular ingestion of these extensively heated products assist in outgrowing egg or cow’s milk allergy.

About the authors

  • Joyce Tan is a Paediatric SRMO from John Hunter Children's Hospital, currently on rotation at Gosford Hospital. She has a passion for medical volunteering and enjoys singing alligator goodbye songs with kiddies at her local church.


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