Photoallergy can be defined as an acquired altered reactivity of the skin presumably dependent on the development of circulation antibodies or cell-mediated immune responses. In contrast to phototoxic reactions, photoallergic responses are uncommon and consist clinically of immediate urticarial or delayed papular and eczematous lesions. Histologically the immediate responses show dermal edema and vasodilatation. The delayed reactions are characterized primarily by perivascular round-cell infiltrates in the dermis. Photoallergy can be induced by exogenous photosensitizers which may arrive at the skin through contact or systemic pathways and the reactions are delayed in nature. Photoallergy not related to exogenous photosensitizers may be immediate (solar urticaria) and possibly delayed (polymorphous light eruptions) in nature.
Epstein JH. PhotoallergyA Review. Arch Dermatol. 1972;106(5):741-748. doi:10.1001/archderm.1972.01620140081025