Allergic hypersensitivity skin reactions following sun exposure.

Photoallergic dermatoses are skin lesions following sun exposure, characterized by activation of immunological mechanisms, involving photosensitizers and photoallergens that can cause photosensibilization in some individuals. In this group of disorders, photoallergic contact dermatitis should be accentuated. It manifests as contact allergic dermatitis on sun-exposed skin areas, following direct contact with photoallergens during UV exposure (predominantly UVA). Under influence of light, photosensitizers get activated, followed by fusion with cutaneous proteins which renders them to complete antigens, and consequential initiation of immunological mechanisms with resulting pathological skin lesions. The most common photoallergens are: sulfonamide antibiotics, phenothiazines, and halogenated salicylanilides. Photoallergic dermatoses are comprised of several disorders, although the causative photoallergen remains unknown (e.g. solar urticaria, polymorphous light eruption and hydroa vacciniforme). Solar urticaria is a rare, acute urticarial reaction on both sun-exposed and covered skin areas, which appears soon after exposure to sun or artificial lighting. Polymorphous light eruption is a relatively common polymorphous skin eruption, which usually appears in spring. Its pathogenesis is unknown, presumably photoallergic reaction. Hydroa vacciniforme is a rare photodermatosis of unknown etiology, which usually presents in summer-time. It is characterized by vesicobullous eruptions, with residual nonesthetic varioliform scarring.