In photoallergic drug photosensitivity, the chemical agent (drug) present in the skin absorbs photons and forms a photoproduct; this photoproduct then binds to a soluble or membrane-bound protein to form an antigen. Since photoallergy depends on individual immunologic reactivity, it develops in only a small percentage of persons exposed to drugs and light.
Formation of photoproduct that conjugates with protein producing an antigen. The action spectrum involved is almost always UVA.
Both phototoxic and photoallergic reactions occur in sun-exposed areas of skin, including the face, V of the neck, and dorsa of the hands and forearms. The hair-bearing scalp, postauricular and periorbital areas, and submental portion of the chin are usually spared. A widespread eruption suggests exposure to a systemic photosensitizer, whereas a localized eruption indicates a reaction to a locally applied topical photosensitizer.
More chronic exposure results in erythema, lichenification, and scaling.
Photosensitizing drugs may also cause a lichen planus*like eruption in sun-exposed areas.
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