Food Allergy

Almost any food or food additive can cause an allergic reaction, but the most common triggers include

Cross-reactivity between food and nonfood allergens exists, and sensitization may occur nonenterally. For example, patients with oral allergies (typically, pruritus, erythema, and edema of the mouth when fruits and vegetables are eaten) may have been sensitized by exposure to pollens that are antigenically similar to food antigens; children with peanut allergy may have been sensitized by topical creams containing peanut oil used to treat rashes. Many patients who are allergic to latex are also allergic to bananas, kiwis, avocados, or a combination.

Food allergies are more common among children whose parents have food allergies, allergic rhinitis Allergic Rhinitis Allergic rhinitis is seasonal or perennial itching, sneezing, rhinorrhea, nasal congestion, and sometimes conjunctivitis, caused by exposure to pollens or other allergens. Diagnosis is by history... read more , or allergic asthma.

In general, food allergy is mediated by IgE, T cells, or both:

Symptoms and signs of food allergies vary by allergen, mechanism, and patient age.

The most common manifestation in infants is atopic dermatitis Atopic Dermatitis (Eczema) Atopic dermatitis is a chronic relapsing inflammatory skin disorder with a complex pathogenesis involving genetic susceptibility, immunologic and epidermal barrier dysfunction, and environmental... read more alone or with gastrointestinal (GI) symptoms (eg, nausea, vomiting, diarrhea). Children usually outgrow these manifestations and react increasingly to inhaled allergens, with symptoms of asthma and rhinitis; this progression is called atopic march. By age 10 years, patients rarely have respiratory symptoms after the allergenic food is eaten, even though skin tests remain positive. If atopic dermatitis persists or appears in older children or adults, its activity seems largely independent of IgE-mediated allergy with a dominance of T-cell–mediated reactions, even though atopic patients with extensive dermatitis have much higher serum IgE levels than atopic patients who are free of dermatitis.

When food allergy persists in older children and adults, the reactions tend to be more severe (eg, explosive urticaria, angioedema, even anaphylaxis). In a few patients, food (especially wheat and shrimp) triggers anaphylaxis only if they exercise soon afterward; mechanism is unknown. Food may also trigger nonspecific symptoms (eg, light-headedness, syncope). Occasionally, cheilitis, aphthous ulcers, pylorospasm, spastic constipation, pruritus ani, and perianal eczema are attributed to food allergy.

T-cell–mediated reactions tend to involve the GI tract, causing symptoms such as subacute or chronic abdominal pain, nausea, cramping, and diarrhea.

Severe food allergy is usually obvious in adults. When it is not or when it occurs in children (the most commonly affected age group), diagnosis may be difficult, and the disorder must be differentiated from other GI problems. Diagnosis of celiac disease Diagnosis Celiac disease is an immunologically mediated disease in genetically susceptible people caused by intolerance to gluten, resulting in mucosal inflammation and villous atrophy, which causes malabsorption... read more is discussed elsewhere.

Testing (eg, allergen-specific serum IgE testing, skin testing) and elimination diets are most useful in diagnosing IgE-mediated reactions. Patients should keep a food diary, meticulously listing everything they consume and any adverse effects they have (particularly timing in relation to food consumption), to help guide decisions regarding elimination of suspect foods.

If a food reaction is suspected, one of the following is done:

In either case, a positive test does not confirm a clinically relevant allergy. Both tests can have false-positive or false-negative results. Skin testing is generally more sensitive than the allergen-specific serum IgE test but is more likely to have to false-positive results. The skin test provides a result within 15 to 20 minutes, much more quickly than the allergen-specific serum IgE test.

If either test is positive, the tested food is eliminated from the diet. If eliminating the food relieves symptoms, the patient is reexposed to the food (preferably in a double-blind test) to see whether symptoms recur (oral challenge testing). (See also the National Institute of Allergy and Infectious Diseases (NIAID) medical position statement: Guidelines for the diagnosis and management of food allergy in the United States.)

If skin testing is not available to confirm food allergy or results are inconclusive, the next options may include one or both of the following:

For the latter diet, no foods or fluids may be consumed other than those specified. Pure products must always be used. Many commercially prepared products and meals contain an undesired food in large amounts (eg, commercial rye bread contains wheat flour) or in traces as flavoring or thickeners, and determining whether an undesired food is present may be difficult.

A discussion with the patient and observations from the patient's food diary can help with the choice of the initial elimination diet. If no improvement occurs after 1 week of the initial diet, another diet should be tried; however, T-cell–mediated reactions may take weeks to resolve. If symptoms are relieved and if patients have less severe symptoms, one new food is added and eaten in large amounts for > 24 hours or until symptoms recur. But if patients have particularly severe symptoms, small amounts of the food to be tested are eaten in the clinician’s presence, and the patient’s reactions observed.

Aggravation or recrudescence of symptoms after addition of a new food is the best evidence of allergy.

Treatment of food allergies consists of eliminating the food that triggers the allergic reaction. Thus, diagnosis and treatment overlap. When assessing an elimination diet’s effect, clinicians must consider that food sensitivities may disappear spontaneously.

Patients with severe food allergies should be advised to carry antihistamines to take immediately if a reaction starts and a prefilled, self-injecting syringe of epinephrine to use when needed for severe reactions. Antihistamines have little value except for acute reactions causing urticaria and angioedema.

Oral cromolyn appears to effectively lessen the allergic reaction to food.

Long-term corticosteroid treatment is useful for symptomatic eosinophilic enteropathy.

For many years, avoiding feeding young infants allergenic foods (eg, peanuts) has been recommended as a way to prevent food allergies. However, one study (1 Prevention references Food allergy is an exaggerated immune response to dietary components, usually proteins. Manifestations vary widely and can include atopic dermatitis, gastrointestinal or respiratory symptoms... read more ) showed that early introduction and regular consumption of food that contains peanuts can prevent peanut allergy in infants at high risk of developing this allergy (eg, infants with egg allergy or eczema). Thus, more clinicians are moving away from infant food restrictions based on allergy concerns and permitting or encouraging consumption of such foods. Current guidelines recommend not delaying the introduction of more foods—more than what is usually recommended for all infants—into the diet of infants, including high-risk infants. Thus, most recommendations are to introduce foods such as egg and peanuts at about age 4 to 6 months (2 Prevention references Food allergy is an exaggerated immune response to dietary components, usually proteins. Manifestations vary widely and can include atopic dermatitis, gastrointestinal or respiratory symptoms... read more ).

The following English-language resource may be helpful. Please note that THE MANUAL is not responsible for the content of this resource.