Fonacier is head of the allergy section of the Division of Rheumatology, Allergy & Immunology at Winthrop University Hospital in Mineola, NY. Her talk, titled “Food Allergy and Atopic Dermatitis: Generating a Common Approach with the Dermatologist,” covered many well-known techniques to diagnose food allergy. She also devoted a large section to a controversial test: the atopy patch test.
What stood out for me was the role of contact dermatitis in atopic dermatitis. According to Fonacier, contact dermatitis, a specific type of allergic reaction to substances such as certain metals and fragrances, affects roughly two-thirds of young (infant to teenage) patients with chronic eczema. Rash and inflammation from contact dermatitis can intensify AD and change the long-term course of the disease, presumably for the worse--by exposing you to more allergens and pathogens.
While contact dermatitis often affects the hands, arms, and face, a systemic exposure to an allergen to which a person has a contact allergy can affect skin over the entire body. Metals, fragrances, and other substances that cause contact dermatitis are often present in foods. When a person eats a food containing a substance to which he or she has a contact allergy, it can manifest as a body-wide skin inflammation.
So how is this different from “classic” food allergy?
The differences lie in the timing and pathway of the reaction. “Type I” allergens such as wheat, soy, milk, and the usual culprits provoke a specific type of allergy, initiated by IgE-class antibodies, that appears over 30 minutes to two hours.
“Type IV” allergens—and I am not even sure I am using the correct term for these substances—cause a “delayed-type hypersensitivity” reaction driven by T cells that manifests hours or days later.
|Nickel. You probably don't want to eat this stuff. |
But it's in your cutlery and tofu.
Nickel is overwhelmingly found in soy and a small number of other foods, according to Fonacier’s slides. (That means that you can have a type IV allergy to soy but not necessarily test positive on a skin prick test or IgE assay.) Balsam of Peru, in contrast, is found in a wide variety of foods. Check out slide 15 of the presentation—see spices, citrus, tomatoes? I have no idea how balsam of Peru ends up in citrus peel or tomatoes; maybe it's an agriculture or food industry thing [update: Fonacier says that tomatoes contain chemicals similar to those in balsam of Peru].
Now, how to diagnose contact dermatitis? Patch testing, in which a nurse applies an array of patches containing potential allergens to a patient’s back. About two days later, an allergist looks for inflamed spots, and the corresponding patches indicate which items you should avoid.
|Balsam of Peru. Appearing soon in toothpaste near you. |
(And spices, and citrus, and tomatoes, and fragrances...)
The advantages of the APT are that it apparently can predict type IV allergies to cow’s milk, egg, and wheat pretty well. The disadvantages are that it takes a long time and the person observing the results has to be well-trained. According to one slide, an NIAID expert panel recommends that the APT not be routinely used in the clinic because it is not as reliable as oral food challenges.
So when does Fonacier recommend using the APT? If a patient has a history of severe and persistent AD, and skin prick and IgE tests have been done, and no trigger has been identified—or if there are multiple instances of IgE reactions that have no apparent connection to AD—then it's time to try the APT.