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photopatch.eu > glossary
Basic facts
Photoallergy is a relevant burden to public health, and an exciting scientific challenge. As many as 5% of the general population have experienced episodes of photodermatoses (light-induced skin diseases, not to be confused with sunburn). Among these patients, 8% are diagnosed with photoallergic contact dermatitis. These figures have been confirmed in both European (Neumann et al. 2000) and American (Fotiades et al. 1995) populations.
The most frequent photosensitizers are substances abundant in our close surroundings: preservatives, perfumes and cosmetics, systemic and topical drugs. Paradoxically, also sunscreens are frequent causes of photocontact allergic dermatitis.
Photopatch testing is the gold standard in the diagnosis of photoallergic contact dermatitis. Therefore, every dermatologist and allergist should be acquainted with this method.
Photoallergy
Photoallergic reaction is a hypersensitivity reaction initiated by specific immune reaction to substances generated or activated by visible light (VL), ultraviolet radiation (UVA, more rarely UVB), and least frequently by infrared radiation (IR). It seems that the prefix "photo" seem to refer to the mechanisms of generation/activation of the allergen or hapten, rather than some exceptional mechanism of specific immune response. The mechanisms of photoallergic reactions seem identical with those of "classic" allergy. Until now, photoallergic reactions fitting to Gell & Coombs type I and IV allergic reactions have been well documented (Table 1).
| Type | Description | Respective photodermatosis | Postulated role of irradiation | Spectrum |
|---|---|---|---|---|
| I | Immediate hypersensitivity, IgE-mediated | Solar urticaria | Initiation of photochemical reactions that result in generation of allergens from endogenous substances or xenobiotics | UVA, VL, UVB, IR |
| II | Cytotoxic reactions | ? | - | - |
| III | Immune complexes | ? | - | - |
| IV | Delayed hypersensitivity (contact allergy) | Photoallergic contact dermatitis | Initiation of photochemical reactions that result in generation of haptens from endogenous substances or xenobiotics; Initiation of coupling of haptens with carrier proteins to form full allergenic molecule | UVA (rarely UVB) |
Spiewak R. Photoallergic diseases - diagnosis and treatment. Alergol Immunol 2008; 5 (3-4): 101-105.
Photopatch testing
Diagnosis of photoallergic contact dermatitis requires respective modification of patch tests, i.e. irradiation of tested skin area with UV. Typically, UVA (wavelength 320-400 nm) is used; in rare cases UVB (290-320 nm) is necessary for the initiation of allergic reaction. The UVA dose used at photopatch testing is 5-10 J/cm2 or, alternatively, 1/2 of Minimal Erythema Dose (MED) determined individually for the tested person [53]. The haptens tested are applied in double sets, with only one being irradiated. While interpreting photopatch test results, both sets of haptens are compared: the "bright" side (patch tests irradiated with UV) with "dark" side (not exposed to UV). A positive result on the "bright" side with a negative result on the "dark" side suggests photoallergy, equal responses on both sides - "classic" contact allergy.
Spiewak R. Patch testing for contact allergy and allergic contact dermatitis. Open Allergy J 2008; 1: 42-51.
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photopatch.eu > glossary
Krakow (Cracow)
Krakow (Cracow)
Krakow (Cracow)
European Society of Contact Dermatitis