- Delayed-type (type IV) hypersensitivity reaction that may develop secondary to a variety of topical agents or drugs
- Clinically resembles allergic contact dermatitis, but the rash is confined to sun-exposed areas, as the responsible agent becomes immunogenic following exposure to light
- Acute to subacute spongiotic dermatitis
- There is often a predominantly superficial dense perivascular lymphocytic infiltrate with endothelial swelling and mural edema.
- If the causative agent is present in sufficient quantities, there may be associated phototoxic features, including:
- Keratinocyte necrosis
- Architectural disarray and dysmaturation
- Hyperpigmentation with transepidermal elimination of melanin