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Allergy/Immunology - Food Allergy - Fast Facts | NEJM Resident 360
Adverse food reactions are untoward responses following the ingestion of a food and can be divided into food allergies (immunologically mediated) and all other reactions (nonimmunologic). Adverse food reactions are common, but nonimmunologic reactions to food are more common than true immunologically mediated food allergies. Nonimmunologic reactions may include gastrointestinal disorders (e.g., lactase deficiency), toxic reactions (including scromboid poisoning), intolerance (e.g., tyramine-containing foods), psychological reactions (food phobias or aversions), or accidental contamination, and these are not covered in this guide.
Food allergies can be divided into IgE-mediated or non–IgE-mediated processes.
IgE-Mediated Food Allergies
Presentation
IgE-mediated food allergy occurs within minutes to 2 hours after food ingestion. The most commonly implicated foods include cow’s milk, eggs, peanuts, soy, tree nuts, fish, shellfish and wheat.
Symptoms may include:
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cutaneous reactions: urticaria, angioedema, flushing
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gastrointestinal manifestations: nausea, vomiting, diarrhea, abdominal pain
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oral symptoms: tongue, lip, or perioral edema and pruritus
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respiratory symptoms: bronchoconstriction and wheezing
Diagnosis
A thorough history is essential and should include temporal association, reproducibility, and clinical symptoms, as described in the following table:
(Source: Food Allergy. N Engl J Med 2008.)
Testing
Confirmation and assessment of food-specific IgE-mediated food allergy can be performed by skin-prick testing (SPT) or blood test for food specific IgE.
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Skin prick testing has a high negative predictive value (generally >90%). However, the positive predictive value can vary greatly depending on age, wheal size, and food tested.
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In vitro tests are very sensitive and have a high false-positive rate. Thus, lab testing should only be performed to evaluate specific foods when there is high clinical suspicion.
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Specific IgE testing can also be helpful when skin testing results are equivocal, to evaluate patients who cannot be skin tested and to trend sensitization over time, especially in children who are likely to outgrow food allergies and become candidates for oral food challenge. Food allergy guidelines do not recommend broadly testing a whole “panel” of foods.
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Specific IgG testing is not a clinically relevant tool for diagnosing food allergies and should not be ordered.
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Oral food challenges can be performed for the diagnosis of food allergies and can help evaluate the development of tolerance.
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Oral food challenges should only be performed in a clinic with trained staff who are knowledgeable and comfortable with the treatment of anaphylaxis and where medications and supplies for emergency resuscitation are immediately available (most often in an allergy-immunology clinic).
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Treatment
Treatment for IgE-mediated food reactions includes avoidance of the food allergens and prompt treatment of reactions caused by accidental ingestion. Treatment of acute reactions depends on the severity of symptoms and progression.
Anaphylaxis requires prompt treatment with epinephrine (see How to Use an EpiPen). Emergency medical services should be called, and the patient should be transported to the nearest hospital (see the section on Anaphylaxis in this rotation guide.)
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Even after initial treatment, symptoms of anaphylaxis may recur several hours after initial reaction (biphasic or late-phase reactions).
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Risk factors for fatal food-induced anaphylaxis are outlined in the following table:
(Source: Food Allergy. N Engl J Med 2017.)
Although many people outgrow allergies with time, allergy to certain foods (e.g., peanuts, tree nuts, and shellfish) often persists through adulthood.
Prevention
In the past, infants at high risk for food allergy and pregnant and lactating mothers were advised to exclude allergenic foods in their diets. However, a recent randomized control trial found that early introduction of peanuts in children at high risk for allergy significantly reduced the risk of developing peanut allergy. Guidelines sponsored by the National Institute of Allergy and Infectious Diseases now recommend introduction of peanuts at age 4 to 6 months in infants at high risk of peanut allergy.
The following table outlines age of onset, cross-reactivity between common food allergies, and usual age of resolution.
(Source: Food Allergy. N Engl J Med 2008.)
The following table outlines the recommended approaches for the evaluation of children with severe eczema, egg allergy, or both before peanut introduction.
Recommended Approaches for Evaluation of Children with Severe Eczema and/or Egg Allergy Before Peanut Introduction
(Reprinted from Addendum guidelines for the prevention of peanut allergy in the United States: Report of the National Institute of Allergy and Infectious Diseases-sponsored expert panel. Ann Allergy Asthma Immunol 2017. Copyright (2017), with permission from Elsevier.)
Management
The following table outlines the current recommendations for management of food allergy.
(Source: Food Allergy. N Engl J Med 2017.)
Several different forms of immunotherapy are currently being investigated to promote desensitization or tolerance to foods through daily low-dose administration.
(Source: Food Allergy. N Engl J Med 2017.)
In January 2020, the FDA approved the first drug for treatment of peanut allergy in children aged 4-17. Palforzia (peanut [Arachis hypogaea] allergen powder-dnfp) introduces peanut in small increasing doses to induce desensitization in peanut-allergic patients.
Oral Allergy Syndrome (Pollen–Food Allergy Syndrome)
Oral allergy syndrome is suspected when a patient with pollen allergy reports oropharyngeal pruritus and edema following exposure to classical culprit foods including fresh fruits, vegetables, and nuts. The reaction is due to cross-reactant, pollen-related proteins in these foods causing a contact reaction when ingested in uncooked preparations. Reactions are generally isolated to the oropharynx, although systemic manifestations have been reported. Any systemic symptoms should prompt allergy testing to exclude a potentially life-threatening IgE-mediated food allergy. Patients with oral allergy syndrome can generally prevent symptoms by peeling, cooking, baking, or even gently microwaving culprit foods to denature the proteins sufficiently to prevent cross-reaction.
(Source: Food allergy: A Practice Parameter Update-2014. J Allergy Clin Immunol 2014.)
Galactose-α-1,3-galactose (α-Gal) Syndrome
Mammalian meat allergy, also known as the “alpha-gal syndrome” is an IgE-mediated food allergy that develops after a tick bite, most commonly the lone-star tick, which is native to the southeastern United States. This syndrome is unique in that, while the symptoms are identical to other IgE-mediated reactions, symptoms onset is delayed, occurring 4-6 hours after ingestion of pork, beef, lamb or other mammalian products (e.g., gelatin or dairy products). Individuals with this syndrome can also experience reactions to the cancer drug cetuximab. Treatment is identical to other IgE-mediated reactions and strict avoidance of mammalian meat is recommended.
Non–IgE-Mediated Food Allergies
Non–IgE-mediated food allergies present as subacute or chronic gastrointestinal-tract or cutaneous symptoms. Non–IgE-mediated food allergy disorders include food protein–induced enterocolitis syndrome (FPIES), food protein–induced enteropathy, food protein–induced proctitis and proctocolitis, and food-induced pulmonary hemosiderosis. Celiac disease is not classically considered a food allergy, but it is caused by a non–IgE-mediated immune reaction to gluten, a food protein.
Mixed IgE- and Non–IgE-Mediated Reactions
Some food allergy disorders have both IgE- and non–IgE-mediated components (e.g., atopic dermatitis, eosinophilic esophagitis, and eosinophilic gastroenteritis. Food allergies may exacerbate atopic dermatitis, especially in young children with severe allergy. Eosinophilic gastrointestinal disorders are characterized by postprandial GI dysfunction and eosinophilic infiltration of the intestinal tract on biopsy.