Structure and function of the skin

This chapter discusses the structure and function of normal skin and skin appendages, and the basics of immune mechanisms that mainly relate to the skin. The major dermal components are collagen fibers, elastic fibers, blood vessels, and the components that maintain the skin, such as the extracellular matrix. Skin consists of a three-layer structure of epidermis, dermis, and subcutaneous tissue. The cells in the epidermis are keratinocytes, melanocytes, and Langerhans cells. Keratinocytes account for 95% of all epidermal cells. The remaining 5% consists of melanocytes, Langerhans cells, and Merkel cells, which are respectively involved in melanin formation, antigen presentation, and sensation. The most important role of melanin is to confer protection to the skin from UV irradiation and thus to reduce the occurrence of malignant tumors and sunlight injury. Keratinocytes, in addition to their role in cornification, play a crucial part in skin immunity. They produce and secrete various cytokines to stimulate immunocompetent cells.