Immune responses to photohaptens: implications for the mechanisms of photosensitivity to exogenous agents.

A photohaptenic moiety is one of the salient properties of photoallergic chemicals. Clinically, it is likely that photohaptens are the main causative substances in photoallergic contact dermatitis and drug photoallergy. Photohaptens bind covalently to protein under exposure to ultraviolet A light (UVA). Because of this photocoupling ability, cells are easily photoderivatized with a photohapten by UVA irradiation, becoming immunogenic photohapten-modified cells. Langerhans cells photomodified with a photohapten can stimulate immune T cells when photomodification is performed with a non-phototoxic dose of UVA.