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What is allergy?

Allergy is not a disease itself, but a mechanism leading to a disease. 


A more current definition could be ‘an immunological hypersensitivity that can lea to a variety of different diseases via different patho-mechanisms with different approaches in diagnosis, therapy and prevention."

Allergy therefore presents as a number of diseases.


allergy is defined as a variety of different diseases


  • Immunological-mediated and allergen-specific hypersensitivity
  • Seen in almost every organ, especially skin and mucous membranes
  • Have different path mechanisms and
  • With different approaches in diagnosis, therapy and prevention


allergy is associated with significant morbdiity

  • Persistent symptoms of AR impair sleep and reduce productivity.
  • Recurrent hospital admissions for asthma cause high absenteeism.
  • Restrictive diets in food allergy cause social exclusion and malnutrition.



any substance stimulating the production of IgE or a cellular immune response. Allergens are usually proteins, but not always. They can for example be carbohydrates.



Normal response to a stimulus.



Abnormal strong response to a stimulus.



Production of IgE antibodies (detected by serum IgE assay or skin prick test) after repeated exposure to an allergen.



A hypersensitivity reaction initiated by a specific immunological mechanism, that is IgE-mediated (e.g. peanut allergy) or non-IgE-mediated (e.g. milk allergy). 



A personal or familial tendency to produce IgE in response to exposure to potential allergens. Atopy is strongly associated with asthma, allergic rhinitis, eczema and food allergy. 



A serious allergic reaction with bronchial, laryngeal and cardiovascular involvement that is rapid in onset and can cause death. 



A substance, whether processed, semi-processed or raw intended for human consumption (including liquids), and any substance used in the manufacture, preparation of treatment of food. 


Food allergy:

A immunologically mediated adverse reaction to foods


Allergy presentation


  • Allergic rhinitis
  • Allergic conjunctivitis
  • Asthma
  • Atopic dermatitis / eczema
  • Urticaria (HIVES)
  • Insect allergy
  • Drug allergy
  • Food allergy


allergic diseases evolve in a characteristic manner with age

The predominant pre-school allergic conditions are eczema and food allergy, both of which decrease in incidence with increasing age as the diseases improve spontaneously.


Asthma predominates in school-aged children and then improves in many and thus decreases in prevalence to adult levels.


Rhinitis and conjunctivitis are rare in young children and evolve through childhood and continue to increase in prevalence into adulthood. We call this change with age the ‘Allergic march’.


Why is allergy important?

1. Allergy is common. For example, in the UK allergic rhinitis affects up to 30% of adults, asthma is diagnosed in more than 1 in 10 children and food allergies are present in about 6% of children and 1-2% of adults. Nearly half of British adults will suffer from at least one allergic condition.

2. Allergy is associated with significant morbidity. The persistent symptoms of allergic rhinitis impair sleep and affect daytime function with reduced productivity in adults and poorer school performances in children. Asthma is a common cause of hospital admissions with one child being admitted to hospital every 20 minutes in the UK. Food allergies, although not symptomatic with effective dietary avoidance, also significantly impair quality of life not only of the affected individual but the family as well. Careful dietary exclusions for example require longer times for shopping to read ingredient labels and impact of social activities limiting eating out, going to parties and so on.

3. Allergy can be fatal. Every year about 1200 people die from asthma in the UK. Admissions to anaphylaxis have increased more than 600% in the 20 years to 2012, although thankfully there has not been a rise in mortality.



Food allergy is defined as

‘an adverse health effect arising from a specific immune response that occurs on exposure to a food. The definition encompasses responses that are classified as IgE-mediated or non-IgE mediated'


Food intolerance

is by contrast the numerous and frequently reported adverse responses to foods that do not involve the immune response


So we classify adverse reactions to food as follows:

1. Non-immune mediated or primary food intolerances

2. Immune mediated or food allergy and Coeliac disease


Non-immune mediated or primary food intolerances.

These are either:

a. Food characteristics such as reactions to pharmacologically active food components (e.g. caffeine) or illness in response to toxins from microbial contamination or to scromboid fish toxin.

  • This is a reaction that results from eating spoiled oily fish (mackerel, tuna) where excess histamine, produced from fish decay, producing symptoms similar to allergy.

b. Host characteristics include

  • metabolic disorders such as lactose intolerance and psychological or neurological responses such as food aversion or rhinorrhoea caused by spicy foods.


2. Immune mediated or food allergy and Coeliac disease.

Food allergy is classified by immune mechanism into IgE and non-IgE mediated types. These have classical presentations but are further classified into a number of conditions depending on clinical picture and severity.


How does food allergy present?

The two main phenotypes of food allergy are

  1. immediate-onset or IgE-mediated and
  2. delayed-onset or non-IgE-mediated. 


IgE mediated

  • immediate


Non IgE mediated



Food allergy presentation: Immediate onset of IgE-mediated food allergy phenotypes



  • Urticaria/angioedema
    • Acute (rarely chronic) hives and swelling with gastro- intestinal but not respiratory or CVS symptoms. (any)
  • Anaphylaxis
    • Rapidly progressive, potentially fatal, multiple organ system reaction with respiratory symptoms and can include cardiovascular. (nuts, fish, shellfish, milk, eggs
  • Food- associated exercise induce anaphylaxis (FRIES)
    • Food triggers anaphylaxis only if ingestion is followed temporally (within 2 hours) by exercise.
      (wheat, shellfish, celery)
  • Pollen food syndrome
    • Pruritus and mild oedema confined to the oral cavity (lips, tongue, mouth and throat) uncommonly progressing. Associated with hay fever (raw fruit and vegetables)


Food allergy presentation: Delayed onset of IgE-mediated food allergy phenotypes



  • Proctocolitis
    • Passage of bright red blood in mucousy stools in otherwise asymptomatic infants. (milk, through breast feeding)
  • Enterocolitis
    • Multiple and varying gastrointestinal symptoms including feed refusal, persistent vomiting, abdominal cramps, loose and frequent stools and constipation. (milk, eggs, wheat)
  • Eosinophilic oesophagitis
    • Symptoms from oesophageal inflammation and scarring of feeding disorders, reflux symptoms, vomiting, dysphagia and food impaction. (milk, eggs, wheat)
  • Food protein-induced entero-colitis syndrome
    • Primarily affects infants. Profuse vomiting leading to pallor, lethargy and possibly shock; diarrhoea in 25%. (milk, soya, rice, wheat, meat)


food allergy march

The age at which food allergies present are influenced mainly by individuals diet. The natural history of food allergies varies as, as already mentioned, food allergy resolves with increasing age. Presentation might be divided into those presenting in infancy (milk, egg and peanut) or in early childhood as the child’s diet becomes and more diverse (soya, wheat, tree nuts, fish, shellfish, sesame, kiwi fruit). Pollen food syndrome is the commonest food allergy in adults. It usually manifests in adolescence and is as a result of cross-reactivity between the pollens of fruit, vegetables and nuts and pollens causing hay fever e.g. birch. Heat or stomach acids easily denature these allergens, so symptoms occur only in unprocessed food where that food has had mucosal contact. On entering the stomach the allergen is denatured preventing absorption and systemic symptoms. The natural history of foods is either to improve or resolve or to persist. Examples of allergies that improve are milk, egg, wheat and fruit, and of those that persist are peanut, tree nut, seed, fish and shellfish allergies


summary of IgE vs non-IgE mediated reaction


Effects of high temperature and food matrix on food proteins.

The ability of food allergens to induce symptoms is influenced by their epitope structure and consequent heat stability. Allergens comprising sequential epitopes that are not damaged by heat tend to be heat stable, whilst those dependent on the three-dimensional structure of the protein, conformational epitopes, will be altered or destroyed by heat and lose their allergenic potential. Protein interactions with other ingredients such as proteins, fats and sugars in processed foods are also important, in general resulting in decreased availability of protein for interactions with the immune system


Examples of the heat and matrix effect are milk and egg.

Baked milk (i.e. milk in processed biscuits) has lower allergenicity and availability to the immune system so can be used to reintroduce milk back in to the diet of children with milk allergy expected to be resolving. Heating raw milk does not seem to reduce allergenicity sufficiently for clinical use.

Another possibly better example is hen’s egg. There are 5 main proteins in eggs, 4 of which are heat labile – the heat stable and thus immuno-dominant protein ovomucoid comprises about 10% of egg proteins. A chard boiled egg or well-cooked scrambled egg will therefore have about 80-90% less allergenicity than raw egg (used in mayonnaise). The allergenicity is further reduced by decreased availability to the immune system in baked egg (e.g. in wheat). Baked egg is thus used as the initial food in assessing and treating resolving egg allergy.