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The skin's most important function is to form an effective barrier between the “inside” and the “outside” of the organism. Life on dry land requires the presence of a barrier to regulate water loss and prevent desiccation, commonly referred to as the “inside–outside” barrier. Additionally, skin provides an “outside–inside” barrier to protect against mechanical, chemical, and microbial assaults from the external environment (Fig. 47-1).1 To perform these functions the epidermis undergoes keratinization, a process in which epidermal cells progressively mature from basal cells with proliferative potential to the lifeless, flattened squames of the stratum corneum (SC) (Fig. 47-2). Both the SC and the deeper skin layers protect the skin from mechanical forces, ultraviolet (UV) radiation, cold and hot temperatures, and invasion of chemicals and microbes. To effectively perform this multiplicity of functions, the skin contains different types of barriers. The physical barrier consists mainly of the SC, but the nucleated epidermis, in particular the tight junctions, provides another important barrier component. The chemical/biochemical (antimicrobial) barrier consists of lipids, acids, lysozymes, and antimicrobial peptides (discussed in Chapter 10). The humoral and cellular immune system provides a barrier to infectious disease, but immune hyperactivity may lead to allergy (Table 47-1).
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