Photoallergen

Photoallergen refers to a chemical substance that becomes capable of triggering an allergic skin reaction only after exposure to light. Photoallergic contact dermatitis is a form of delayed-type (type IV) hypersensitivity that requires prior sensitization and subsequent irradiation, typically with ultraviolet A (UVA) or, less often, visible light. It is distinct from phototoxic reactions, which are non-immunologic and can occur on first exposure.

Mechanism and presentation. A photoactivated molecule forms a complex with skin proteins, creating a hapten that can be recognized by the immune system. This leads to sensitization of T cells. On re-exposure and light activation, a dermatitis develops at sun-exposed sites such as the face, neck, and forearms. The reaction tends to be eczematous, pruritic, and may persist or recur with light exposure and the presence of the allergen.

Common sources. Fragrances and fragrance ingredients are a frequent cause of photoallergic dermatitis. Essential oils and natural products containing furocoumarins (for example bergamot oil) can be photosensitizing. Some sunscreen ingredients and other cosmetic filters, including certain benzophenones and cinnamates, have been reported as photoallergens. Chemical hair dyes (such as those containing para-phenylenediamine) and other occupational exposures can also cause photocontact reactions.

Diagnosis and management. Diagnosis relies on clinical history and specialized testing, notably photopatch testing, which applies allergens to the skin with and without UVA exposure and reads reactions over several days. Management centers on avoiding identified photoallergens, strict photoprotection, and treating inflammation with topical corticosteroids when needed. If a drug is implicated, discontinuation or substitution under medical supervision is advised.