photoallergic

Photoallergic reaction, or photoallergy, is an immune-mediated skin response that occurs when a chemical substance absorbs ultraviolet light and elicits a T-cell–mediated allergic reaction in the skin. It is a type IV hypersensitivity reaction that requires prior sensitization to a photosensitizer, followed by subsequent UV exposure. The eruption typically appears on sun-exposed areas and presents as an itchy, eczematous dermatitis that may resemble other forms of dermatitis. By contrast, phototoxic reactions can occur in anyone after sufficient UV exposure to a photosensitizing substance and do not require prior sensitization.

Causes commonly involve components of cosmetics, personal care products, and medications. Photoallergens include fragrance ingredients, preservatives, certain sunscreen agents, and plant-derived compounds such as furocoumarins (psoralens) found in citrus fruits and bergamot oil. Some systemic drugs (for example, certain antibiotics, diuretics, and nonsteroidal anti-inflammatory drugs) can contribute to photoallergic dermatitis when present in the skin and exposed to light.

Diagnosis is based on history and specialized testing, notably photopatch testing, where suspected substances are applied to the skin on irradiated and non-irradiated sites to identify causative agents.

Management centers on avoidance of the identified allergen and protection from sun exposure. Treatments include topical corticosteroids to reduce inflammation and itching, oral antihistamines for pruritus, and, in severe cases, brief courses of systemic corticosteroids. Prognosis improves with strict avoidance, though reactions can recur with re-exposure.