Food allergy
Exam Tips
- In OSCE history, timing is key: IgE reactions usually occur within minutes to 2 hours; delayed GI-predominant symptoms suggest non-IgE mechanisms.
- A positive SPT or sIgE indicates sensitization, not necessarily clinical allergy; correlate strictly with history.
- For anaphylaxis stations, first-line treatment is IM adrenaline in the thigh; antihistamines and steroids are not first-line life-saving therapy.
- Know high-risk allergens for severe reactions: peanut, tree nuts, fish, shellfish; always ask about coexisting asthma control.
- Pollen-food syndrome usually causes localized oral symptoms to raw fruit/vegetables in pollen-sensitized patients; systemic reactions are less common but possible.
Definition
Food allergy is an adverse reaction to food caused by an immune mechanism, most commonly IgE-mediated and immediate in onset after exposure to a specific food. It is distinct from food intolerance because symptoms are reproducible with re-exposure and may involve skin, gastrointestinal, respiratory, and cardiovascular systems, with potential progression to anaphylaxis.
Pathophysiology
IgE-mediated food allergy occurs after sensitization to a food allergen, with allergen-specific IgE binding to mast cells and basophils; re-exposure causes rapid mediator release (for example histamine, leukotrienes), producing symptoms within minutes to 1-2 hours. Non-IgE disease is mainly T-cell/cell-mediated and typically delayed, often with gastrointestinal-predominant presentations (for example food protein-induced enterocolitis syndrome). Mixed mechanisms are seen in conditions such as cow's milk protein allergy and eosinophilic gastrointestinal disease. Cross-reactivity explains syndromes such as pollen-food syndrome (for example birch pollen with raw apple/hazelnut) and latex-fruit syndrome. See Figure from page X.
Risk Factors
- Personal history of atopy (especially early-onset or severe eczema in infancy)
- Existing confirmed food allergy (risk of additional food allergies)
- Family history of food allergy or atopy (parent/sibling affected)
- Egg allergy in infancy (associated with later peanut allergy)
- Asthma, especially poorly controlled asthma (risk factor for severe/fatal reactions)
- Adolescence/young adulthood (higher risk of severe accidental reactions)
- Impaired skin barrier and topical exposure to food proteins (for example peanut oil-containing emollients)
Clinical Features
Symptoms
- Rapid-onset oral pruritus, lip/tongue swelling, throat tightness after trigger food
- Generalized itching, flushing, urticaria
- Nausea, colicky abdominal pain, vomiting, diarrhoea
- Cough, wheeze, chest tightness, breathlessness, hoarse voice
- Dizziness, presyncope/syncope, confusion (suggesting systemic compromise)
- Pollen-food syndrome: brief itch/tingle in mouth and oropharynx after raw fruit/vegetables
Signs
- Urticaria and/or angioedema (lips, eyelids, face)
- Wheeze, stridor, tachypnoea, increased work of breathing
- Hypotension, tachycardia or bradycardia, prolonged capillary refill in severe reactions
- Cyanosis, collapse, reduced consciousness in anaphylaxis
- In non-IgE presentations (often infants): pallor, lethargy, dehydration, faltering growth
Investigations
Management
Lifestyle Modifications
- Provide strict, individualized allergen avoidance advice including label reading and cross-contamination prevention
- Issue a written emergency action plan for home/school/work, including when to use adrenaline auto-injector and when to call 999
- Refer to specialist allergy services for diagnostic uncertainty, previous anaphylaxis, multiple food allergies, or nutritional concern
- Dietitian-supported elimination and reintroduction planning to prevent unnecessary long-term restriction
- Optimize coexisting asthma control and educate on higher anaphylaxis risk if asthma is poorly controlled
Pharmacological Treatment
Adrenaline (first-line for anaphylaxis)
- Adrenaline IM 1 mg/mL (1:1000): adults and children >12 years 500 micrograms (0.5 mL), repeat every 5 minutes if needed
- Adrenaline IM 1 mg/mL (1:1000): children 6-12 years 300 micrograms (0.3 mL), repeat every 5 minutes if needed
- Adrenaline IM 1 mg/mL (1:1000): children 6 months-6 years 150 micrograms (0.15 mL), repeat every 5 minutes if needed
- Auto-injectors: 150 micrograms, 300 micrograms, or 500 micrograms device strength according to age/weight and specialist advice
No absolute contraindication in anaphylaxis. Give IM into anterolateral thigh promptly; delayed adrenaline increases mortality. Patients on beta-blockers may have more refractory reactions and need urgent specialist/emergency management.
H1 antihistamines (adjunctive, not rescue for airway/circulation)
- Cetirizine oral: adults and adolescents 10 mg once daily
- Cetirizine oral: children 6-11 years 5 mg twice daily (or 10 mg once daily depending on product)
- Chlorphenamine oral: adults 4 mg every 4-6 hours (max 24 mg/day)
Use for cutaneous symptoms (itch/urticaria) after stabilization. Do not use antihistamines as sole treatment for suspected anaphylaxis.
Bronchodilator for persistent wheeze (adjunctive)
- Salbutamol inhaler 100 micrograms per puff: 4-10 puffs via spacer, repeated according to response
Adjunct for bronchospasm only; does not replace adrenaline in anaphylaxis.
Complications
- Anaphylaxis, including life-threatening airway, breathing, and circulatory compromise
- Accidental exposure with recurrent emergency presentations
- Nutritional deficiency and faltering growth in children on broad exclusion diets
- Psychological morbidity (anxiety, hypervigilance, reduced quality of life, social restriction)
- School/work participation limitations and stigma
Prognosis
Prognosis varies by allergen and comorbidity. Milk, egg, soy, and wheat allergy in childhood often improve over time, whereas peanut, tree nut, fish, and shellfish allergies are more likely to persist. Severe reaction risk is higher with asthma and prior severe episodes; regular review is needed to reassess tolerance, update emergency plans, and adjust auto-injector prescribing.
Sources & References
🏥BMJ Best Practice(1)
💊BNF Drug References(1)
- Chlorphenamine maleate[management.pharmacological]
✅NICE Guidelines(1)
- Food allergy[overview]
📖Textbook References(3)
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 670)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 828)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 112)[context]