Photoallergens

Photoallergens are substances that, after exposure to light, particularly ultraviolet A (UVA), become capable of triggering an immune-mediated contact dermatitis in individuals who have previously become sensitized. The reaction is a delayed-type (type IV) hypersensitivity and usually affects sun-exposed skin where the allergen and light interact.

Mechanism involves the absorption of light by the chemical, generating reactive photo-products that bind to skin proteins and form hapten-protein complexes. These complexes are processed by antigen-presenting cells and presented to T cells, leading to a localized inflammatory response upon re-exposure to the allergen and light.

Common sources of photoallergens include fragrances and fragrance ingredients, preservatives, and certain sunscreen filters. Early and well-documented examples include para-aminobenzoic acid (PABA) derivatives and other older sunscreen agents; more recently reported agents include some sunscreen filters such as certain benzophenone-related compounds. Plant-based products containing furocoumarins (for example certain essential oils and bergamot-derived components) can also act as photoactive sensitizers. Medication-associated photoallergic reactions can occur with topical or occasionally systemic drugs when combined with light exposure.

Diagnosis relies on clinical history and specialized testing. Photopatch testing, in which suspected allergens are applied to the skin and then exposed to UVA, helps identify photoactivated allergens, often alongside standard patch tests.

Management centers on avoidance of the identified photoallergen and strict photoprotection, including protective clothing and sun avoidance when possible. Inflammation is treated with appropriate topical or, if needed, systemic anti-inflammatory therapy. Education on sun exposure patterns and ongoing monitoring are important for affected individuals.