Food allergy

Definition

Food allergy is an abnormal immune response to specific proteins in foods, in which the immune system mistakenly identifies these proteins as harmful and triggers a reaction. Unlike food intolerance, food allergy involves immune mechanisms (often IgE-mediated) and can be life-threatening in severe cases such as anaphylaxis.

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Epidemiology

• Prevalence: ~6–8% of children and ~2–4% of adults

• Onset: often in childhood, but can occur at any age

• Higher prevalence in individuals with atopic diseases (asthma, eczema, allergic rhinitis)

• Common in families with allergic conditions (genetic predisposition)

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Pathophysiology

Food allergy reactions are mediated by immune pathways:

1. IgE-mediated (immediate-type hypersensitivity) – most common

   • Upon first exposure, the immune system produces allergen-specific IgE antibodies

   • Subsequent exposure causes IgE to bind allergens, triggering mast cell and basophil degranulation

   • Release of histamine and other mediators leads to rapid onset symptoms (minutes to hours)

2. Non-IgE-mediated – delayed reactions mediated by T cells

   • Symptoms develop hours to days after ingestion

   • More commonly involve the gastrointestinal tract and skin

3. Mixed IgE and non-IgE mechanisms – e.g., eosinophilic oesophagitis

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Common Allergenic Foods

• Children: cow’s milk, eggs, peanuts, soy, wheat, tree nuts

• Adults: peanuts, tree nuts, fish, shellfish

• Others: sesame seeds, celery, lupin, mustard

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Clinical Presentation

Onset: Minutes to hours after ingestion (IgE-mediated) or delayed for non-IgE-mediated

Symptoms:

• Skin: urticaria (hives), angioedema, eczema flare

• Gastrointestinal: abdominal pain, nausea, vomiting, diarrhoea

• Respiratory: nasal congestion, sneezing, wheezing, cough, dyspnoea

• Cardiovascular: dizziness, hypotension, tachycardia (in severe reactions)

• Anaphylaxis: severe, rapid-onset, multi-system reaction that can be fatal without treatment

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Diagnosis

1. History and Clinical Presentation

   • Detailed symptom onset, food exposure, and reaction patterns

   • Family history of allergies or atopy

2. Allergy Testing

   • Skin Prick Test (SPT) – immediate hypersensitivity to specific allergens

   • Serum-specific IgE testing – measures IgE levels to specific foods

   • Oral Food Challenge – gold standard, performed in a medical facility due to risk of severe reaction

3. Elimination Diet

   • Removal of suspected allergen for 2–6 weeks followed by controlled reintroduction

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Management

1. Allergen Avoidance

• Complete avoidance of confirmed allergen

• Careful label reading for packaged foods

• Awareness of cross-contamination risk in restaurants and food preparation

2. Emergency Management of Acute Reactions

• Mild symptoms (localised rash, mild GI upset):

  • Non-sedating antihistamines such as cetirizine 10 mg orally once daily or loratadine 10 mg orally once daily

• Severe reaction / Anaphylaxis:

  • Epinephrine (adrenaline) – first-line

    • Adults: 0.3–0.5 mg intramuscularly (1:1000) into the mid-thigh, repeat every 5–15 min if needed

    • Children: 0.01 mg/kg IM (max 0.3 mg per dose)

  • Call emergency services immediately

  • Place patient in supine position with legs elevated unless breathing difficulty requires sitting

  • Administer supplemental oxygen and IV fluids if available

  • Adjunctive medications:

    • Chlorphenamine 4 mg orally every 4–6 hours (max 24 mg/day) for skin symptoms

    • Prednisolone 30–50 mg orally (short course) to reduce risk of biphasic reaction (not first-line)

3. Long-term Management

• Prescription of epinephrine auto-injector (e.g., EpiPen) for at-risk patients, with training on use

• Medical alert bracelet or card indicating allergen

• Education for patient, family, and caregivers on avoidance and emergency response

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Special Considerations

Oral Allergy Syndrome (OAS)

• Cross-reactivity between pollen allergens and proteins in fresh fruits/vegetables

• Symptoms limited to mouth and throat (itching, tingling, mild swelling)

• Usually mild but can progress in rare cases

Exercise-induced Food Allergy

• Reaction occurs only when exercise follows ingestion of specific food (e.g., wheat-dependent exercise-induced anaphylaxis)

Food Protein-induced Enterocolitis Syndrome (FPIES)

• Severe, delayed, non-IgE-mediated allergy in infants and young children

• Profuse vomiting, diarrhoea, dehydration hours after ingestion

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Prognosis

• Many childhood food allergies (e.g., milk, egg) resolve with age

• Allergies to peanuts, tree nuts, fish, and shellfish often persist for life

• Risk of accidental exposure remains even with strict avoidance

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Prevention

• Early introduction of allergenic foods in infancy (from 4–6 months) may reduce risk in high-risk infants

• Exclusive breastfeeding for first 4–6 months is recommended for overall infant health

• Education on food labelling laws and allergen disclosure