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Photosensitivity reactions may be either phototoxic or photoallergic. Phototoxic reactions to chemicals can occur in anyone and essentially are exaggerated sunburn responses, characterized by erythema and urticaria; a delayed sunburn-type pattern (within 72 hours); or a delayed melanin hyperpigmentation reaction (72-96 hours). Phototoxic photosensitivity is dependent on the concentration of photosensitizing compounds found in the drug or plant. The pathogenesis is formation of free radicals, reactive oxygen species, or other toxic photoproducts. Photoallergic reactions, on the other hand, are far less common than phototoxic reactions, depend on individual immunological reactivity, and occur only in previously sensitized individuals. The rash also appears different; it is papular, vesicular, or eczematous in appearance, resembling allergic contact eczematous dermatitis or lichen planus-like eruptions. The pathogenesis involves the conjugation of a photoproduct with a protein, producing an antigen.
Adapted from: Fitzpatrick TB. Color Atlas and Synopsis of Clinical Dermatology. 3rd ed. New York: McGraw-Hill; 1997.