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The surface of the skin is covered in antimicrobial peptides and lipids, some of which originate from the sebaceous gland located near the hair follicle. 1ĭiagrammatic representation of human skin barrier immunity. The deepest layer of the skin is the subcutaneous layer, which consists of subcutaneous fat and connective tissue. In addition to the extracellular matrix, the dermis contains structures such as blood vessels, lymphatics, nerves, sweat glands and pilosebaceous units. The dermis predominantly consists of the extracellular matrix, such as collagen, which is made by fibroblasts. The second layer is the dermis, a thicker layer (up to 3–4 mm depending on body site), which has a relatively low cell volume compared with the epidermis.
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The epidermis does not have a blood supply of its own, but instead is nourished from blood vessels below. The epidermis is mainly composed of keratinocytes however, there are also melanocytes, which provide a barrier from ultraviolet (UV) radiation through expression of melanin. The stratified epithelium provides a watertight barrier from the external environment and prevents excessive water loss from the body. The skin consists of three layers: the top layer is the epidermis, a thin layer (approximately 0♱ mm thick) of stratified squamous epithelium, composed of four strata of keratinocytes in progressive stages of differentiation. The skin is the outermost layer of the body with an extensive surface area of approximately 1♸ m 2, and is the first line of defence against a multitude of external pathogens. Together, these alterations result in decreased barrier immunity in the elderly, explaining in part their increased susceptiblity to cancer and infections. In addition, the skin immune composition is altered with reduced Langerhans cells, decreased antigen‐specific immunity and increased regulatory populations such as Foxp3 + regulatory T cells. As we age, the skin structure changes with thinning of the epidermis and dermis, increased water loss, and fragmentation of collagen and elastin. Ageing results in an increased incidence of cancer and skin infections.
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We also discuss specialized immune cells that are resident in steady‐state skin including mononuclear phagocytes, such as Langerhans cells, dermal macrophages and dermal dendritic cells in addition to the resident memory T cells. In this review, we discuss how the structural cells such as keratinocytes, fibroblasts and adipocytes contribute to barrier immunity. The structure of the skin and its cellular composition work in harmony to prevent infections and to deal with physical and chemical challenges from the outside world. The skin also has important homeostatic functions such as reducing water loss and contributing to thermoregulation of the body. The skin is the outermost layer of the body with an extensive surface area of approximately 1♸ m 2, and is the first line of defence against a multitude of external pathogens and environmental insults.