Phototoxicity and photoallergy in man

Photosensitivity is the broad term used to denote adverse reactions to the sun. These reactions may be phototoxic or photoallergic. Phototoxic reactions are much more common than photoallergic reactions. Phototoxic reactions may be photodynamic or nonphotodynamic in nature, the primary difference being that photodynamic reactions require oxygen. Porphyrins are examples of molecules which produce photodynamic responses. The 8-methoxypsoralen photobinding to DNA is an example of a nonphotodynamic reaction. The UVB sunburn response is the most common phototoxic reaction, and it appears to be nonphotodynamic in nature. Photoallergy is uncommon. As with all allergies it is an acquired, altered reactivity dependent on an antigen-antibody or cell-mediated hypersensitivity response. Solar urticaria due to UVB rays is an example of a circulating antibody-dependent reaction, and allergic photocontact dermatitis induced by the halogenated salicylanilides is an example of a cell-mediated response. The mechanisms of these reactions are discussed.