Allergy & Immunology

Food allergy

A reproducible immune-mediated reaction to a specific food, ranging from immediate IgE-mediated urticaria or anaphylaxis to delayed non-IgE-mediated skin and gastrointestinal disease; it is distinct from non-immune food intolerance.

In a nutshell

Food allergy is a reproducible immune reaction to a specific food. Immediate IgE-mediated disease causes urticaria, angioedema, vomiting, wheeze or anaphylaxis; delayed non-IgE-mediated disease commonly causes eczema or gastrointestinal symptoms. Diagnose from an allergy-focused history, use focused IgE testing for suspected IgE-mediated disease and a supported elimination/reintroduction pathway for non-IgE disease. Confirmed allergy needs safe avoidance, dietetic support and a written emergency plan; do not describe withdrawn Palforzia guidance as a routine new-start option.

Classic presentation

A child develops reproducible lip swelling, urticaria and vomiting within minutes of eating a suspected food, often with eczema or another atopic condition.

Key points

The history determines whether an immediate IgE-mediated, delayed non-IgE-mediated, mixed or non-immune process is most likely.
A positive specific-IgE or skin-prick test indicates sensitisation and only supports allergy when it matches the clinical history.
There are no validated routine tests for non-IgE-mediated food allergy; use a defined elimination and planned reintroduction pathway.
Do not use serum IgG panels, Vega testing, kinesiology, hair analysis or other unvalidated tests to diagnose food allergy.
Treat anaphylaxis immediately with IM adrenaline; do not delay for antihistamines, steroids or testing.
Extensive avoidance can cause nutritional deficiency, faltering growth and anxiety; use dietetic support and reassess tolerance.
NICE TA769 has been withdrawn: no new UK Palforzia starts should be described, although existing treatment may continue under the current transition arrangement.

First-line investigation

An allergy-focused history, followed by focused specific-IgE blood testing or skin-prick testing when immediate IgE-mediated allergy is suspected.

Management

Treat anaphylaxis without delay

For airway, breathing or circulation compromise after a suspected food exposure, give IM adrenaline immediately, follow the RCUK pathway and repeat after 5 minutes if needed; antihistamines and steroids do not replace adrenaline.^8,4,9

Build the diagnosis from timing and phenotype

Record the exact food, amount, timing, preparation, reproducibility, co-factors and symptoms by organ system; distinguish immediate IgE-mediated disease from delayed non-IgE disease, mixed patterns and food intolerance.^1,2
Use focused specific-IgE or skin-prick testing only when the history suggests IgE-mediated disease; interpret results with the clinical history and avoid broad panels.^1,2

Make avoidance nutritionally safe

Use a time-limited elimination and planned reintroduction for suspected non-IgE disease, with dietetic support for children, multiple-food exclusion, breast-feeding, formula changes, faltering growth or nutritional risk.^1,2
Provide label-reading, cross-contamination, school and childcare advice; maintain tolerated foods and treat asthma, eczema, rhinitis, reflux or constipation according to their own pathways.^1,3,12,6

Refer and plan prevention

Refer suspected anaphylaxis, severe or recurrent reactions, diagnostic uncertainty, significant nutritional restriction, faltering growth or need for oral challenge to specialist allergy care; provide two trained auto-injectors where risk warrants them.^1,10,11
Describe Palforzia accurately: NICE TA769 has been withdrawn and no new people should start it in the UK under the current information; existing patients may continue under the stated transition arrangements.¹⁴

Reassess tolerance and safety

Review growth, nutrition, anxiety, accidental exposures, auto-injector expiry and training, school or childcare plans and whether tolerance has developed; update the written plan after a specialist challenge or reintroduction.^1,10,11

Exam traps

Food allergy is immune-mediated; food intolerance is not and does not cause IgE-mediated anaphylaxis.
A positive allergy test alone does not diagnose clinical allergy.
Do not order broad food-allergy panels or unvalidated IgG tests.
Do not use a home oral challenge for suspected IgE-mediated allergy; specialist supervised challenge may be required.
Do not continue a diagnostic elimination diet indefinitely without planned reintroduction and nutritional review.
Do not call Palforzia a routine new-start UK option while NICE TA769 remains withdrawn.

Illustrations

IgE-mediated mast cell degranulationA diagram of sensitisation with food-specific IgE binding mast cells, followed by allergen cross-linking on re-exposure and mediator release causing urticaria, angioedema, gastrointestinal symptoms or anaphylaxis.PassFinals · original

IgE-mediated versus non-IgE-mediated food allergyA comparison of IgE-mediated allergy, non-IgE-mediated allergy and non-immune food intolerance, showing mechanism, timing, typical symptoms and the appropriate diagnostic approach.PassFinals · original

Urticarial rashConfluent blanching erythematous wheals showing a typical immediate skin feature of IgE-mediated allergy, with a reminder that the presence or absence of rash does not define anaphylaxis.Psixtras, Wikimedia Commons · CC0

AngioedemaPeriorbital angioedema as a possible feature of an immediate food-allergic reaction, with airway assessment and emergency escalation shown as the priority.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.