Food allergy is an adverse reaction due to a specific immune response occurring reproducibly upon exposure to certain foods. It occurs minutes to hours after food consumption.
Immunological mechanisms can be IgE mediated, non-IgE mediated or mixed IgE and non-IgE mediated.
It may be life-threatening and is the most common cause of anaphylaxis in children.
It should be differentiated from food intolerance in which adverse reactions from exposure to food arise from non-immunological mechanisms.


Specific Food Allergies (FA)

  • Common food allergens are often glycoproteins
  • Although any food can evoke an IgE-mediated reaction, some are more allergenic than others
  • For infants & young children, common allergens are cow’s milk, egg, peanut, tree nuts, wheat & soybean; for adults, these are peanuts, tree nuts, fish, crustaceans & mollusks

Cow’s milk allergy (CMA)

  • Affects 2-3% of the general population
  • May be present at birth & even in infants exclusively breastfed; not all adverse reactions to cow’s milk are allergic in nature
  • 90% of those allergic to cow’s milk will react to goat’s or sheep’s milk due to high degree of cross-reactivity
  • Exclusive breastfeeding is recommended until 4-6 months of age, unless breastfeeding is contraindicated for medical reasons
  • To meet nutritional requirements of infants w/ CMA, it is recommended to prescribe a substitute formula until at least 2 years of age
    • Extensively hydrolyzed formulae are the first choice, except in patients w/ anaphylaxis & eosinophilic esophagitis
    • Soy-based formulae should not be given in infants <6 months of age
  • Factors that predict the acquisition of tolerance include declining serum IgE levels, absence of asthma or allergic rhinitis & never being formula-fed
  • In approximately 80% of affected infants, tolerance is obtained by 3-5 years of age

Egg allergy

  • Strongly associated w/ atopic dermatitis
  • MMR vaccine may be safely given to patients w/ egg allergy
  • Patients 18-49 years w/ mild-severe egg allergy should receive influenza vaccination
  • Patients w/ egg allergy who need influenza vaccine should be referred to a clinical facility experienced in anaphylaxis management
  • In approximately 67% of affected children, tolerance is obtained by 5 years of age

Peanut allergy

  • Usually life long, often severe & potentially fatal
  • Patients can generally tolerate other beans, including soy
  • Avoidance of other legumes is not needed as clinical cross-reactions are uncommon; however, tree nuts should still be avoided

Tree nut allergy

  • Eg walnut, cashew, hazelnut, pecan, almond, pistachio
  • May be severe & potentially life-threatening; allergy is usually life long
  • Common among children w/ peanut allergy (30-50%)


Classification of Food Allergies (FA) [based on underlying immunological mechanism]

IgE-mediated food allergy

  • More common in children, involving up to 6% of children <5 years old
  • Frequently affects children w/ atopic dermatitis (AD)
    • Studies found greater likelihood of FA in younger children w/ more severe AD
  • Allergic reaction occurs immediately, w/in minutes up to 1-2 hours following exposure to the causative food allergen
  • Reactions recur upon repeated exposure; small amount of food may cause severe reactions
  • Oral allergy syndrome (Pollen-food syndrome)
    • Seen in patients allergic to pollens that cross react w/ those found in fresh vegetables & fruits
    • Oropharyngeal contact w/ food allergen results in itching, tingling & angioedema of the lips, tongue, palate & throat
  • Gastrointestinal anaphylaxis
    • Arising most commonly after ingestion of the following proteins: milk, egg, peanut, soy, cereal & fish
    • Nausea, cramping abdominal pain, vomiting, diarrhea or combination of these conditions may occur after ingestion of allergen
  • Non-gastrointestinal manifestations include cutaneous, respiratory reactions & systemic anaphylaxis
  • Life-threatening reactions are associated w/ peanuts, nuts, fish & shellfish

Non-IgE mediated food allergy

  • Studies have shown that T cells may mediate the pathogenesis of some subtypes of this food allergy
  • Often presents w/ abdominal symptoms (eg vomiting, abdominal pain, hematochezia)
  • Symptoms occur several hours after ingestion of allergen
  • Food protein-induced enterocolitis syndrome
    • Symptoms develop w/in 1-4 hours of ingestion of causative allergen
    • Typically occurs in the first few years of life & may manifest as blood-streaked stools in infants who are frequently breastfed
  • Food protein-induced allergic proctocolitis
    • Is a common transient disease of infancy, usually resolving w/in the first 2 years of life
    • Occurs w/ ingestion of cow’s, soy or human breast milk in infancy
    • May manifest as mucoid, blood-streaked stools, diarrhea, vomiting & failure to thrive
  • Food protein-induced enteropathy syndrome
    • Characterized by generalized malabsorption of nutrients, resulting in chronic diarrhea, weight loss & growth failure among infants; symptoms usually disappear by 2-3 years of age
    • Most often caused by milk allergy but soy, chicken, fish & rice may also act as allergens
  • Allergic contact dermatitis
    • Cell-mediated allergic reaction to foods or food contaminants
    • Allergic reactions initiated by contact w/ chemical components in the food

Mixed IgE & non-IgE mediated food allergy

  • Suspected when symptoms occur in a more chronic nature, do not resolve quickly, or are not closely associated w/ ingestion of the food allergen
  • Allergic eosinophilic enteropathies
    • Characterized by intense eosinophilic infiltration of specific organ involved (eg esophagus, stomach, small intestine)
    • Symptoms include vomiting, anorexia, abdominal pain, hematemesis, reflux, weight loss, anemia & failure to thrive


  • A key part of the diagnostic work-up is an allergy focused clinical history
  • Obtain the following information:
    • Suspected food/foods that caused the adverse reaction & the symptoms produced
    • Time interval between ingestion of food & development of symptoms
    • Amount of food that caused the reaction
    • Reproducibility of the reaction in relationship to food ingestion
    • Other cofactors [eg exercise, alcohol, infection, drugs (eg NSAIDs)]
    • Length of time from previous reaction
    • Details of previous medications for the presenting symptoms, including response to these treatments
    • For children, obtain the child’s feeding history (if breastfed, consider mother’s diet), including age of weaning
  • Important to identify comorbidities, risk & predisposing factors for possible recurrence [eg history of previous anaphylactic reactions, history of atopy, asthma even if well-controlled, adolescence, family history of atopic disease or any type of food allergy (FA)]
    • Family history remains the most practical & useful tool in identifying allergy-prone infants
    • Presence of allergic diseases in one or both parents & in a sibling increases the likelihood of allergy in a child
  • More useful in the diagnosis of immediate food-induced allergies than delayed reactions
  • Although useful, history alone is not considered diagnostic of FA

Physical Examination

  • A focused physical examination (PE) can provide signs consistent w/ food-induced adverse reaction; however, no specific findings in PE are diagnostic
  • Distinguish & recognize features of anaphylaxis from less severe allergic reactions to food
  • In patients w/ acute allergic reaction to food, airways should be checked for obstruction from laryngeal edema & bronchospasm

Laboratory Tests

  • Tests are selected based on history, which should suggest the possible allergic mechanism involved
  • Laboratory testing is of limited value in the acute care setting since treatment is of utmost importance

Skin Prick Test (SPT)

  • Also referred to as puncture or epicutaneous test
  • Helps identify foods that provoke IgE-mediated allergic reactions but SPT alone cannot be considered diagnostic
  • Useful in determining the food responsible in IgE-mediated food allergy (FA) in patients w/ confirmed FA
  • Patients should stop taking antihistamines before skin testing to avoid false negative results
    • Long-acting antihistamines should be avoided for 10 days while short-acting antihistamines for 3 days before the skin test
  • Has a low positive predictive value in the initial diagnosis of FA but a high negative predictive value
  • Positive test correlates w/ presence of allergen-specific IgE bound to the surface of cutaneous mast cells
  • Positive SPT is a wheal w/ mean diameter ≥3 mm than the negative control & flare >10 mm
  • Different from intradermal testing, which is associated w/ a greater risk of systemic adverse allergic reactions & is not recommended in the diagnosis of FA

Allergen-specific Serum IgE

  • Standard method to establish allergen sensitization
  • Helps identify foods that provoke IgE-mediated allergic reactions but serum IgE alone cannot be considered diagnostic
  • Useful when SPT cannot be done
    • When patient has extensive dermatitis or has severe dermographism
    • When patient cannot withdraw antihistamine use
    • Patient’s reaction was anaphylactic & there is great risk even for skin testing
    • Unavailability of extract or reagent for skin testing
  • Levels may be used to evaluate whether sensitization to food is increasing, stable or waning over time
    • May predict the likelihood of a reaction but not its severity

Food Elimination Diets

  • May be useful in identifying foods causing some non-IgE mediated food allergies
  • Considered diagnostic & therapeutic in patients w/ adverse reactions to foods, regardless of mechanism involved
  • When combined w/ a convincing history, food elimination diets may be enough to make a diagnosis of FA in several food-induced allergic disorders
  • If instituted for longer periods, ensure that the patient is able to meet nutritional requirements to avoid deficiencies

Oral Food Challenge

  • Considered the most definitive test for food allergy
  • May be open, single-blind, or double-blind & placebo-controlled
  • Double-blind, placebo-controlled food challenge (DBPCFC) is the gold standard in diagnosing FA
    • Disadvantages include cost, potential for severe allergic reactions & is time-consuming
  • Open or single-blind food challenges may be used in clinical setting since DBPCFC can be very expensive & inconvenient
    • When results are negative, may be diagnostic in ruling out FA
    • When results are positive, may be considered diagnostic in patients supported by history & laboratory data
  • Test should be performed under medical supervision to document the dose that provokes the reaction & to administer treatment should adverse reactions, including anaphylaxis, occur

Food-induced Anaphylaxis

  • A serious allergic reaction that has rapid onset & may be life-threatening
    • Prompt assessment & treatment are critical as death can ensue rapidly
  • Clinical features include those seen in food allergy plus respiratory compromise &/or hypotension
  • Food allergy (FA) is the most common cause of anaphylaxis
  • Individuals at high risk for fatal food-induced anaphylaxis include adolescents, those w/ history of allergic reactions, peanut/tree nut allergy, asthma, & those w/o skin manifestations
  • In cases of food-dependent, exercise-induced anaphylaxis, reactions occur depending on the temporal association between food ingestion & exercise (usually w/in 2 hours)
  • Nuts, fish, shellfish, cow’s milk, soy & eggs most commonly trigger anaphylaxis, but fruits & vegetables have also been implicated
  • Please see Anaphylaxis Disease Management Chart for more details
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