Food allergy can be classified into IgE mediated and non IgE mediated reactions. Usually this is easily identified: if a typical history is present with supportive IgE based tests, then the diagnosis is usually secure.
Many non IgE reactions are believed to be T cell mediated. Some reactions involve a mixture of both IgE and non IgE responses and are classified as mixed IgE and non IgE allergic reactions.
IgE mediated food allergy
- Very small amounts of food can elicit significant reactions.
- Very rapid onset of symptoms usually within 20 minutes, but often within 1-2 minutes (World Allergy Organisation definition is within 2 hours).
- It is common occurring in 5-6% of young children In Ireland. The exact incidence is unknown but is likely to be very similar to that in the UK 3-6% of preschool children and 1-2% of older children and adults.
- Most children with food allergy have other atopic conditions, especially eczema occasionally asthma and allergic rhinitis.
- In infants <2 years, food allergy can exacerbate existing eczema but there is no justification for manipulating an infant or child’s diet until skin care with topical steroids and emollients/ointments has been optimised. Dietary manipulation must be short term and under experienced supervision from an allergy team that includes a Dietitian. Children >2 years with eczema should not have dietary manipulation without expert medical assessment.
- Well known common food triggers (milk, egg, peanut, nuts, fish) account for more than 90% of cases.
- The most common food allergies in the first few years of life include milk, egg and peanut. In children >3yrs common food allergies include peanut, tree nuts, fish, shellfish.
- Most children will outgrow allergy to milk or egg. Most children will not outgrow a peanut, tree nut, fish or shellfish allergy.
- Due to the relatively low positive predictive value (50%) of low level positive tests, specific IgE should only be tested for only 2 or 3 suspected foods.
- Negative IgE test results are highly specific and have a negative predictive value of 95% for milk, egg and peanut (Negative results are system specific e.g. 0.1 for some foods or 0.35 KUA/L in Thermofisher’s system)
- Individual allergic proteins of milk and peanut can be tested by using recombinant allergens (e.g. Ara h 2, Bos d 1). At present their contribution to routine clinical care is uncertain but is evolving.
- Urticaria and angioedema on their own are minor symptoms.
- Cough and hoarseness imply upper airway obstruction, are underappreciated and should be treated as severe symptoms.
- Wheeze (even mild), feeling faint are severe symptoms.
- Anaphylaxis is considered a severe food allergic reaction associated with lower respiratory or cardiovascular features.
- Many children with IgE mediated reactions will need to have adrenalin prescribed as part of their care plan.
- Children with both food allergy and asthma are at increased risk of a severe food allergic reaction.
Non IgE mediated food allergy
- Symptoms may not appear for more than 24 hours after exposure. There may be no response on the first day because some of these mechanisms are dose related (unlike immediate IgE mediated reactions).
- A small dose may be tolerated but incremental doses are not tolerated.
- Symptoms are more diffuse and include enteropathies and eczema.
- In infants <2 years, food allergy can exacerbate existing eczema but there is no justification for manipulating an infant or child’s diet until skin care has been optimised. Dietary manipulation must be short term and under experienced supervision from an allergy team that includes a Dietitian. Children >2 years with eczema should not have dietary manipulation without expert medical assessment.
- There is no definitive in vitro test for non IgE mediated food allergies.
- Dietitian supervised exclusion and reintroduction is the only supportable diagnostic and possibly therapeutic intervention. This should be time defined (4-6 weeks duration) and exclude no more than 4 foods.
- Gastrointestinal (GI) syndromes include eosinophilic oesophagitis, which can present with Gastro-oesophageal reflux disease (GORD) like symptoms, and other eosinophilc enteropathies.
- Consider food allergy in children with refusal to feed, severe aversive feeding behaviour, problems progressing the weaning diet, growth faltering etc., especially if they also have eczema that is difficult to control.
- Food allergies are rarely the isolated, removable cause of upper airway symptoms such as chronic rhino-sinusitis or middle ear disease.
Mixed IgE and non IgE symptom clusters (of the above) can occur. Non IgE mediated can convert to IgE mediated allergy and therefore long term follow up is essential.