Food Allergy

Food allergy is an adverse immune mediated reaction to a specific food protein. The mechanism determines the timing, clinical presentation and diagnostic approach.

• IgE mediated: immediate onset, type one hypersensitivity.
• Non IgE mediated: delayed onset, predominantly T cell mediated.
• Mixed.

Food allergy is distinct from food intolerance, which involves no immune mechanism and is typically related to enzymatic or metabolic factors such as lactose intolerance.

Common allergens and natural history

The most frequent culprits are cow's milk, egg, peanut, tree nuts, soy, wheat, fish and shellfish. They account for the large majority of significant food allergy.

Milk and egg allergy in childhood frequently resolve by school age. Peanut, tree nut, fish and shellfish allergy tend to persist into adulthood. This distinction matters for prognostic counselling and for the timing of any reassessment.

IgE mediated reactions develop rapidly, typically within minutes to two hours of ingestion. They may involve:

• cutaneous symptoms (urticaria, angioedema, flushing)
• gastrointestinal symptoms (vomiting, abdominal pain)
• respiratory symptoms (wheeze, stridor)
• in severe cases, anaphylaxis with cardiovascular compromise.

Non IgE mediated reactions develop more gradually, over hours to days. They are predominantly gastrointestinal.

• Food protein induced enterocolitis syndrome: profuse vomiting and lethargy developing hours after ingestion, occasionally with hypotension, in infants.
• Food protein induced allergic proctocolitis: blood streaked stool in an otherwise well infant.
• Eosinophilic oesophagitis: dysphagia, food impaction and reflux like symptoms in older children and adults.

Oral allergy syndrome (pollen food syndrome) presents with rapid onset itching and mild swelling confined to the mouth and throat. It follows ingestion of certain raw fruits or vegetables in patients with pollen sensitisation, reflecting cross reactivity between pollen and food proteins.

It rarely progresses to systemic symptoms, because the responsible proteins are labile to cooking and digestion.

A careful history is central to identifying the likely food and mechanism. Establish the specific food, the time interval to symptom onset, and the consistency of the reaction on repeated exposure.

Also establish any co-factors. Exercise and alcohol can lower the threshold for reaction in some patients, as in food dependent exercise induced anaphylaxis.

A detailed clinical history remains the foundation of diagnosis. Testing without a compatible history has poor predictive value. It risks false positive results (sensitisation without clinical allergy) and inappropriate, unnecessarily restrictive dietary exclusion.

Skin prick testing or specific serum IgE testing supports the diagnosis of suspected IgE mediated food allergy. The skin prick wheal size or specific IgE level gives probabilistic rather than absolute information about the likelihood of clinical reactivity. Interpret it alongside the clinical history, not in isolation.

Component resolved diagnostics measure IgE to individual protein components within a food. They help distinguish clinically significant primary sensitisation from cross reactive sensitisation of lesser relevance. They are particularly useful in complex cases such as suspected peanut allergy with pollen cross reactivity.

Oral food challenge remains the diagnostic reference standard. It is performed under close medical supervision with resuscitation facilities available. It is used to confirm or exclude allergy where the history and initial testing are inconclusive, or to assess whether a previously diagnosed allergy has resolved.

Non IgE mediated reactions are diagnosed predominantly on clinical grounds and response to an elimination diet with subsequent structured reintroduction. Specific IgE testing is characteristically negative in these conditions.

Endoscopy with biopsy confirms eosinophilic oesophagitis, showing characteristic eosinophilic infiltration of the oesophageal mucosa.

Strict avoidance of the identified causative food is the cornerstone of management for all forms of food allergy. It is supported by detailed dietary education, ideally with dietitian input: reading food labels, understanding cross contamination risk, and awareness of less obvious sources of the allergen.

Patients with a history of, or significant risk factors for, anaphylaxis to a food allergen should be prescribed an adrenaline autoinjector and trained in its use. They should have a written personalised allergy action plan, with clear guidance on recognising and responding to accidental exposure.

Mild IgE mediated reactions confined to cutaneous symptoms may be managed with antihistamines. Any reaction involving respiratory or cardiovascular symptoms requires adrenaline and emergency management as for anaphylaxis.

Non IgE mediated conditions are managed with dietary elimination of the causative food under dietitian supervision. Structured reintroduction over time assesses for resolution, as many non IgE mediated food allergies of infancy resolve with age.

Eosinophilic oesophagitis is managed with one of the following:

• dietary elimination, either targeted to identified triggers or empirical elimination of the most commonly implicated foods
• topical swallowed corticosteroids
• increasingly, dupilumab for moderate to severe or refractory disease.

Endoscopic dilation is reserved for symptomatic oesophageal stricture.

Oral immunotherapy is an increasingly available option for selected food allergies, particularly peanut. It involves gradual, medically supervised escalating exposure to the allergenic food, aiming to induce desensitisation.

It requires ongoing regular exposure to maintain the desensitised state and carries a risk of reactions during treatment. Careful patient selection and specialist delivery are required.

Regular reassessment avoids unnecessarily prolonged dietary restriction once tolerance has developed. This applies particularly to children with allergies that frequently resolve with age (milk, egg). Reassessment uses history, testing and, where appropriate, supervised food challenge.