Phototoxicity. Photo-irritation. Photo-allergy

The cutaneous pathological mechanism following exposure of the skin to UV rays manifests itself in the form of cytotoxicity (phototoxicity), localized irritation (photo-irritation) or type IV cutaneous allergy (photoallergy) (Photo-patch test in humans).

The effect is caused by the photochemical modification of chemical substances within the skin by the action of UV rays. These substances, which have been absorbed by the body either locally in the skin or by other routes, reach the skin surface via the circulating blood and are transformed on the surface by UV rays penetrating the superficial layers of the epidermis. The result is localized cellular damage of varying severity (cytotoxicity), irritation or an allergic reaction, all characterized by localized redness (erythema), thickening of the epidermis (edema) or hyperpigmentation (discoloration).