Skin architecture and function

Abstract

From early life until death, a challenge for the wellbeing of all higher organisms is the detection and destruction of invading microorganisms, and the elimination of cells that undergo malignant transformation. This challenge has been met by defence mechanisms of the immune system, the most basic principle of which is the recognition of antigens. The mammalian immune system comprises innate (including factors such as complement, antimicrobial peptides, cytokines, chemokines, and cells like dendritic cells, macrophages, natural killer cells, polymorphonuclear leukocytes) and adaptive (T and B lymphocytes) functional components. Both possess different types of recognition receptors and differ in the speed in which they reply to a potential danger. Even though distinct, the innate and adaptive immune system interact and can influence the extent and type of their counterpart and act in synergy to defend the host against infection, cancer and autoimmunity. For higher organisms, the skin is the first barrier that protects the body from disorders caused by infectious or chemical agents, thermal and electromagnetic radiation, and mechanical trauma and is critically involved in immune reactivity. Immune responses in the skin involve an armamentarium of immune cells and soluble mediators. Professional antigen-presenting cells such as epidermal Langerhans cells and dermal dendritic cells in combination with T cells and other resident cells orchestrate the decision between immunity and tolerance. Such responses in the skin are part of the systemic immune system. Although the components of the epidermis and dermis work in concert to accomplish immune responses in the skin, the focus of this chapter will mainly be on the cells, receptors and mediators of the epidermal (immunologic) unit, the frontline of immune protection against harmful threats.