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Harrison's Principles of Internal Medicine, 18e
| Part 2. Cardinal Manifestations and Presentation of Diseases > Section 9. Alterations in the Skin > |
Chapter 56. Photosensitivity and Other Reactions to Light
Sections: Solar Radiation, Photoprotection, Phototherapy and Photochemotherapy, Further Reading.
Topics Discussed: photosensitivity disorders.
Excerpt:"Sunlight is the most visible and obvious source of comfort in the environment. The sun provides the beneficial effects of warmth and vitamin D synthesis; however, acute and chronic sun exposure also have pathologic consequences. Few effects of sun exposure beyond those affecting the skin have been identified, but cutaneous exposure to sunlight is the major cause of human skin cancer and can have immunosuppressive effects as well.Skin consists of two major compartments: the outer epidermis, which is a stratified squamous epithelium, and the underlying dermis, which is rich in matrix proteins such as collagens and elastin. Both compartments are susceptible to damage from sun exposure. The epidermis and the dermis contain several chromophores capable of absorbing incident solar energy, including nucleic acids, proteins, and lipids. The outermost epidermal layer, the stratum corneum, is a major absorber of UV-B, and <10% of incident UV-B wavelengths penetrate through the epidermis to the dermis. Approximately 3% of radiation below 300 nm, 20% of radiation below 360 nm, and 33% of short visible radiation reaches the basal cell layer in untanned human skin. In contrast, UV-A readily penetrates to the dermis and is capable of altering structural and matrix proteins that..."
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