The Photobiology of Human Skin

Abstract

1. INTRODUCTION

The skin is the largest organ of the body and the one which is most exposed to external insults, such as chemicals, infecting microorganisms and mechanical trauma. These insults include UV radiation from the sun. The structure of skin is complex: it is composed of three basic layers, the epidermis, dermis and subcutis, each comprising a variety of cell types. It has been recognised (Streilein 1978) that the skin contains its own immune system, first called skin associated lymphoid tissues, which generally acts very effectively to deal with any local disturbances. However UV radiation poses two unique and potentially dangerous consequences for the skin. It induces genotoxic changes, these mutations leading on some occasions to the development of skin cancers, and it can also suppress cell-mediated immune responses to a variety of antigens. The reason for the latter change may be to prevent excessive inflammation in sun-exposed skin but, if it occurs at the same time as, say, an infection or oncogenesis, then there may be disadvantages for the host. Solar UV radiation does not always cause harmful effects in the skin, and one case where it is beneficial is in promoting the synthesis of vitamin D, essential for calcium metabolism and a healthy skeleton. This aspect is covered in Chapter 12.