Chapter 3. Food Allergens
Food allergies are more common in individuals with atopic dermatitis than in non-atopic individuals. Food allergies may induce dermatitis and contribute to severity of skin disease in some patients, while in others urticaria, or non-cutaneous symptoms such as nasal congestion or wheezing are elicited. Placebo-controlled, food challenge studies have demonstrated that foods can induce skin rashes a subset of children with moderate to severe atopic dermatitis. Infants and young children with food allergy generally have positive immediate skin tests or serum IgE directed to various foods particularly egg, milk, wheat, soy and peanut. Although these foods account for approximately 90% of the foods that exacerbate atopic dermatitis, any remaining food including rice or lamb could be a potential culprit.
In patients with severe atopic dermatitis, it may be necessary to hospitalize the patient to clear up their eczema and challenge them to suspected foods before ruling out the possibility of food allergy. In infants with food allergy who are being breast fed, the mother will need to restrict her diet to eliminate potential food allergens which may be transferred through breast milk. Removal of proven food allergens from the patient's diet may lead to significant clinical improvement. It is important for patients to completely avoid implicated foods, as even small amounts of the food allergen can contribute to food-specific IgE synthesis. In the patient with severe atopic dermatitis, it may be difficult to observe worsening of the eczema in response to certain foods until the skin rash has been completely cleared first.
Potential food allergens can be identified by taking a careful history and carrying out selected allergy skin tests. Negative prick skin tests or serum tests for allergen-specific IgE have a high predictive value for ruling out suspected allergens. Positive skin or in vitro allergy tests, particularly to foods, often do not correlate with clinical symptoms and should be confirmed with controlled food challenges or elimination diets. As a rule, extensive elimination diets, which in some cases can be nutritionally deficient, are not necessary. While food challenges have generally been performed to help define clinical allergy, more recently, specific IgE levels to several food allergens by the Pharmacia ImmunoCAP system have been shown to have predictive value in terms of clinical relevance. Serial measurements using this assay have proven to be of value in following the natural history of patients’ food allergies to help determine when a food could be re-introduced or a food challenge performed.