The BAT II Study is a cross-sectional diagnostic study in which children with suspected IgE-mediated allergy to foods (namely cow's milk, egg, sesame and cashew), as defined by a history of an immediate-type allergic reaction to a food or no history of food consumption or the presence of food-specific IgE as documented by skin prick test or serum specific IgE, will undergo a diagnostic work-up to confirm or refute the diagnosis of IgE-mediated food allergy. Participants will be prospectively recruited from specialised Paediatric Allergy clinics in London and will undergo skin prick testing (SPT), specific IgE testing to allergen extracts and allergen components, basophil activation test (BAT) and oral food challenge. The diagnostic accuracy of the BAT and of other allergy tests will be assessed against the clinical gold-standard.

Inclusion Criteria:

1. Children ≥6 months and <16 years old;

2. Suspected IgE-mediated food allergy defined by:

   • History of an immediate-type allergic reaction to a specific food or
   • No history of consumption of the specific food or
   • IgE sensitisation documented by skin prick test (≥1 mm) or serum specific IgE (≥0.10 KU/L);

3. Avoidance of the specific food for at least 2 days prior to blood collection for BAT and specific IgE and prior to the challenge;

4. Informed consent obtained from parent or guardian and assent obtained from the child.

Exclusion Criteria:

1. Clinically significant chronic illness other than atopic diseases;

2. Previous history of severe life-threatening reaction to the suspected food with documented decrease in oxygen saturation (<90%), hypotension (≥20% reduction in systolic blood pressure) and/or admission to intensive care;

3. Unwillingness to comply with study procedures, namely to undergo a diagnostic food challenge;

4. Contra-indication for diagnostic food challenge, namely:

   • Uncontrolled atopic diseases (e.g. eczema, asthma, rhinitis);
   • Chronic medical conditions that pose significant risk in the event of anaphylaxis or treatment of anaphylaxis (e.g. cardiac disease, severe lung disease, pregnancy, mastocytosis);
   • Inability to discontinue medications that might interfere with assessment or safety (e.g. antihistamines, β-agonists, β-blockers, NSAIDs, ACE inhibitor, antacids);
   • Recent (within 7-14 days) treatment with systemic steroids or prolonged high-dose systemic steroids or immunosuppressants;

5. Undergoing treatment with omalizumab, food allergen immunotherapy or other systemic immunomodulatory treatment;

6. Inability to stop anti-histamines prior to SPT.

Children seen in specialized Pediatric Allergy clinics with suspected allergy to one of the foods studied, namely milk, egg, sesame and nuts.