Residency · Residency · Allergy Immunology

Food Allergy - Diagnosis and Management

Epidemiology

IgE-mediated food allergy affects approximately 6 to 8 percent of children and 3 to 4 percent of adults, with prevalence increasing over recent decades. Self-reported food allergy is substantially higher, with approximately 19 percent of adults claiming a food allergy, though only about 50 percent of self-reporters have confirmed allergy upon formal evaluation. The FASTER Act of 2021 designated nine major allergens for mandatory food labeling in the United States: milk, egg, peanut, tree nuts, wheat, soy, fish, shellfish, and sesame. These nine allergens collectively account for approximately 90 percent of IgE-mediated food allergies in the United States. Food allergy-related anaphylaxis generates approximately 30,000 emergency department visits and an estimated 150 deaths per year in the United States.

Classification of Adverse Food Reactions

IgE-Mediated (Immediate)

IgE-mediated food reactions develop within minutes to 2 hours after ingestion and manifest as urticaria, angioedema, vomiting, wheezing, or anaphylaxis. The most commonly implicated foods include peanut, tree nuts, milk, egg, fish, shellfish, wheat, soy, and sesame. The natural history of IgE-mediated food allergy varies markedly by allergen: allergies to milk, egg, wheat, and soy are commonly outgrown, with 50 to 80 percent of children developing tolerance by school age, while allergies to peanut, tree nuts, fish, and shellfish typically persist throughout life.

Mixed IgE and Non-IgE

Mixed mechanisms underlie eosinophilic esophagitis (discussed in a separate lecture) and the food-triggered component of atopic dermatitis, in which food triggers contribute to disease activity in approximately 30 to 40 percent of children with moderate to severe disease.

Non-IgE-Mediated (Cell-Mediated)

Food protein-induced enterocolitis syndrome (FPIES) is characterized by an onset of 1 to 4 hours after ingestion in its acute form, with profuse projectile vomiting, diarrhea, pallor, and lethargy that can progress to hypovolemic shock. Common triggers include milk, soy, rice, oat, and fish, with onset typically in infancy. Skin prick tests and specific IgE are negative, and the diagnosis is clinical. Acute episodes are managed with ondansetron (0.15 mg/kg intravenously or intramuscularly) as a first-line treatment alongside strict avoidance of the trigger food. Most children outgrow FPIES by age 3 to 5 years.

Food protein-induced allergic proctocolitis (FPIAP) presents with blood-streaked stools in an otherwise well-appearing breastfed infant, triggered by milk protein passed through breast milk or present in formula. The condition is benign and resolves with maternal dairy elimination or transition to a hypoallergenic formula. Food protein-induced enteropathy causes chronic diarrhea, malabsorption, and failure to thrive, with histologic findings resembling celiac disease (villous atrophy) but without anti-tissue transglutaminase antibodies.

Pollen-Food Allergy Syndrome (OAS)

Pollen-food allergy syndrome results from IgE cross-reactivity between pollen proteins and homologous proteins in plant-derived foods, producing oropharyngeal symptoms (tingling, itching of the mouth, lips, and throat) upon ingestion of the raw food. Heat-labile proteins such as PR-10 and profilins are denatured by cooking, making cooked forms tolerated. Heat-stable proteins, particularly lipid transfer proteins (LTPs and nsLTPs), can cause systemic reactions and are more prevalent as triggers in Mediterranean countries.

Diagnosis

Clinical History

A thorough clinical history is the foundation of food allergy evaluation and should establish the specific food ingested, the quantity consumed, the preparation method, the timing from ingestion to symptom onset, the reproducibility of the reaction upon subsequent exposures, and the presence of potential cofactors such as exercise (food-dependent exercise-induced anaphylaxis), alcohol, NSAIDs, and menstruation.

Skin Prick Testing (SPT)

Skin prick testing for food allergens uses a wheal at least 3 mm greater than the negative control as the threshold for positivity, indicating sensitization. Skin prick testing has good negative predictive value (approximately 95 percent) but only moderate positive predictive value (approximately 50 percent), reflecting the important distinction that sensitization does not equate to clinical allergy. Larger wheal sizes correlate with higher probability of clinical reactivity; for example, peanut wheals of 8 mm or greater, milk wheals of 8 mm or greater in children, and egg wheals of 7 mm or greater achieve positive predictive values exceeding 95 percent.

Serum Specific IgE (sIgE)

Serum-specific IgE measured by ImmunoCAP is the most widely used in vitro platform. Decision points for 95 percent positive predictive value for clinical reactivity in children have been established for several major allergens: peanut at 14 kU/L or greater, milk at 15 kU/L or greater, egg at 7 kU/L or greater, fish at 20 kU/L or greater, and tree nuts at approximately 15 kU/L (varying by specific nut). Lower values do not exclude allergy and must be interpreted in clinical context. Trends in specific IgE over time provide important prognostic information, as declining levels may predict the development of tolerance.

Component-Resolved Diagnostics (CRD)

Component-resolved diagnostics identifies IgE directed against individual molecular components of food allergens, providing superior diagnostic precision compared with whole-extract testing. For peanut, Ara h 2 (a 2S albumin seed storage protein) is the most specific marker for clinical peanut allergy, with a level above 0.35 kU/L achieving approximately 90 percent positive predictive value. Ara h 1 and Ara h 3 are additional seed storage proteins associated with systemic reactions. Ara h 8, a PR-10 homolog cross-reactive with birch Bet v 1, indicates pollen cross-reactivity associated with oral allergy syndrome rather than systemic peanut allergy. Ara h 9, a lipid transfer protein, is associated with severe reactions predominantly in Mediterranean regions.

For milk, casein (Bos d 8) IgE predicts persistent allergy, while IgE to whey proteins (Bos d 4, Bos d 5) may resolve over time. For egg, ovomucoid (Gal d 1) IgE predicts allergy to cooked egg (as ovomucoid is heat-stable), while ovalbumin (Gal d 2) is heat-labile. For wheat, omega-5-gliadin (Tri a 19) serves as the specific marker for wheat-dependent exercise-induced anaphylaxis. For tree nuts, 2S albumins (Cor a 14 for hazelnut, Jug r 1 for walnut, Ana o 3 for cashew) predict clinical allergy as opposed to PR-10 cross-reactivity.

<image>A visual guide to component-resolved diagnostics for major food allergens. Organized as a grid with columns for peanut, milk, egg, wheat, and tree nut (hazelnut as example). For each food, show individual molecular components in boxes: component name, protein family, molecular weight, heat stability (thermometer icon: stable or labile), and clinical significance (colored: red = systemic reaction risk, yellow = usually OAS only, green = cross-reactive marker without clinical significance). Specific examples: Peanut row shows Ara h 1 (7S globulin, heat-stable, systemic), Ara h 2 (2S albumin, heat-stable, best predictor of clinical allergy - highlighted), Ara h 8 (PR-10, heat-labile, OAS only), Ara h 9 (LTP, heat-stable, severe reactions in Mediterranean). Milk row: casein (heat-stable, persistent allergy) vs whey proteins (heat-labile, may be outgrown). Egg row: ovomucoid (heat-stable, baked egg reactive) vs ovalbumin (heat-labile, baked egg tolerated).</image>

Oral Food Challenge (OFC)

The oral food challenge remains the gold standard for diagnosing food allergy and for monitoring the resolution of established allergy. The procedure involves graded administration of the suspected food under medical supervision, typically employing 5 to 6 escalating doses over 2 hours until a cumulative dose equivalent to an age-appropriate serving is reached (for example, 1 mg, 3 mg, 10 mg, 30 mg, 100 mg, 300 mg, and 1000 mg of food protein). An open challenge, in which both the patient and physician are aware of the food being administered, is adequate for clinical practice. The double-blind placebo-controlled food challenge (DBPCFC) remains the gold standard for research, eliminating bias. The challenge is stopped upon the appearance of objective allergic signs such as urticaria, vomiting, respiratory symptoms, or hypotension. Approximately 2 to 5 percent of oral food challenges result in symptoms requiring epinephrine, necessitating full resuscitation capability. Indications for oral food challenge include confirming or excluding a diagnosis, assessing whether a previously confirmed allergy has resolved, and evaluating tolerance to baked forms.

Early Introduction and Prevention

LEAP Study (Learning Early About Peanut Allergy)

The landmark Learning Early About Peanut Allergy (LEAP) study by Du Toit and colleagues, published in the New England Journal of Medicine in 2015, fundamentally changed the approach to food allergy prevention. This randomized controlled trial enrolled 640 high-risk infants (those with severe eczema and/or egg allergy) aged 4 to 11 months and randomized them to early peanut introduction (Bamba peanut puffs, providing 6 grams of peanut protein per week) versus strict peanut avoidance. The results demonstrated an 86 percent relative reduction in peanut allergy at age 5 (3.2 percent in the introduction group versus 17.2 percent in the avoidance group). The LEAP-On follow-up study confirmed that this protective effect was sustained even after 1 year of avoidance between ages 5 and 6.

Current Guidelines (NIAID Addendum 2017, AAP)

Current guidelines for early allergen introduction stratify infants by risk. High-risk infants (those with severe eczema and/or egg allergy) should begin introduction of peanut-containing foods at approximately 4 to 6 months of age. If the infant has severe eczema, skin prick testing or serum-specific IgE to peanut should be performed before introduction. If the peanut skin prick test wheal is 8 mm or greater, a supervised oral food challenge may be necessary. A wheal of 3 to 7 mm warrants introduction in the office or at home with guidance. A wheal below 3 mm allows home introduction. Moderate-risk infants (mild to moderate eczema) should have peanut introduced around 6 months of age. Low-risk infants (no eczema or food allergy) should have allergenic foods introduced with other complementary foods per family preference. The concept of early introduction is being extended to other allergens, as studied in the EAT trial, which examined randomized early introduction of six allergenic foods.

<image>An infographic summarizing the NIAID Addendum guidelines for early peanut introduction. Three risk tiers shown as traffic lights. HIGH RISK (red tier): severe eczema and/or egg allergy, age 4-6 months - evaluate with SPT or sIgE before introduction, show decision tree based on SPT result (>=8mm: supervised challenge; 3-7mm: introduce in supervised or home setting; <3mm: introduce at home). MODERATE RISK (yellow tier): mild-moderate eczema, age ~6 months - introduce peanut-containing foods at home. LOW RISK (green tier): no eczema or food allergy - introduce with other complementary foods per family preference. Center panel shows LEAP study results: bar graph comparing peanut allergy prevalence at age 5 (3.2% introduction group vs 17.2% avoidance group). Bottom note: "Do NOT delay introduction of allergenic foods; early introduction prevents food allergy."</image>

Management

Allergen Avoidance

Strict avoidance of confirmed food allergens with careful label reading remains the cornerstone of management. The Food Allergen Labeling and Consumer Protection Act (FALCPA) mandates labeling of the top allergens, and the FASTER Act of 2021 added sesame as the ninth major allergen. Advisory labeling such as "may contain" or "processed in a facility with" is voluntary and not standardized, though threshold studies from the VITAL (Voluntary Incidental Trace Allergen Labelling) program are informing the development of standardized reference doses. Comprehensive patient and family education regarding cross-contamination risks, restaurant safety, label reading techniques, and hidden allergen sources is essential.

Emergency Action Plan

Every patient with confirmed food allergy should have a written, individualized emergency action plan for accidental exposure. At least two epinephrine auto-injectors should be prescribed, with thorough training on their use provided to the patient and family members. Indications for epinephrine use include any systemic symptoms, respiratory symptoms, cardiovascular symptoms, or involvement of two or more organ systems. Antihistamines serve as adjuncts for mild cutaneous symptoms only and are not a substitute for epinephrine. Medical alert identification should be recommended.

Baked Milk and Baked Egg

Approximately 70 to 80 percent of milk-allergic children tolerate extensively heated (baked) milk, and a similar proportion of egg-allergic children tolerate baked egg. Extensive heating denatures conformational epitopes present in whey proteins and ovalbumin (which are heat-labile), while linear epitopes in casein and ovomucoid (which are heat-stable) remain intact. Component-resolved diagnostics assists in predicting tolerance: high casein (Bos d 8) IgE predicts reactivity to baked milk, while high ovomucoid (Gal d 1) IgE predicts reactivity to baked egg. Supervised introduction of baked forms through structured ladders of increasingly heated products may accelerate the development of oral tolerance.

Emerging Therapies

Oral immunotherapy is discussed in a dedicated lecture. Omalizumab (anti-IgE) received FDA approval in 2024 for the reduction of allergic reactions to one or more foods, based on the OUTMATCH trial demonstrating that omalizumab reduced the severity of allergic reactions to multiple foods simultaneously. Importantly, omalizumab does not cure food allergy but rather provides a protective buffer against accidental exposure, using the same weight-and-IgE-based dosing table as in asthma. Epicutaneous immunotherapy (EPIT) with the Viaskin Peanut patch has completed phase 3 trials in children aged 4 to 11 years. Sublingual immunotherapy (SLIT) for food allergy remains primarily in the research stage.

Key Clinical Pearls

• Sensitization (positive SPT or sIgE) does NOT equal clinical food allergy; oral food challenge is the gold standard for diagnosis
• Ara h 2 is the most specific component for predicting clinical peanut allergy; Ara h 8 positivity with negative Ara h 2 suggests pollen cross-reactivity, not true peanut allergy
• The LEAP study demonstrated 86% reduction in peanut allergy with early introduction at 4-6 months in high-risk infants
• FPIES presents with profuse vomiting 1-4 hours after ingestion; it is non-IgE-mediated, SPT/sIgE are negative; ondansetron is first-line for acute episodes
• Baked egg/milk tolerance (70-80% of allergic children) is predicted by low ovomucoid (Gal d 1) / low casein (Bos d 8) IgE respectively
• Omalizumab is now FDA-approved for multi-food allergy to reduce severity of accidental exposures; it does not eliminate the allergy
• Declining specific IgE and increasing SPT wheal size-to-sIgE ratio over time may predict developing tolerance

References

1. Sicherer SH, Sampson HA. Food allergy: a review and update on epidemiology, pathogenesis, diagnosis, prevention, and management. J Allergy Clin Immunol. 2018;141(1):41-58.
2. Du Toit G, et al. Randomized trial of peanut consumption in infants at risk for peanut allergy (LEAP). N Engl J Med. 2015;372(9):803-813.
3. Togias A, et al. Addendum guidelines for the prevention of peanut allergy in the United States (NIAID). J Allergy Clin Immunol. 2017;139(1):29-44.
4. Sampson HA, et al. Food allergy: a practice parameter update - 2014. J Allergy Clin Immunol. 2014;134(5):1016-1025.
5. Santos AF, et al. EAACI guidelines on the diagnosis of IgE-mediated food allergy. Allergy. 2023;78(12):3057-3076.

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