The skin includes two main layers, the outer epidermis composed mainly of epithelial cells and, separated by a thin basement membrane, the underlying dermis composed of connective tissue and specialized adnexal structures such as hair follicles and sweat glands. Within both of these layers, a variety of different cell types and their products, composing the cutaneous immune system (Fig. 13-9), provide physical barrier and active immune defense functions against microbes. The skin of an adult is about 2 m2 in area and is the second largest barrier of the body against environmental microbes and other foreign materials. Nonetheless, given its outermost location, the skin is normally colonized by many microbes and is frequently breached by trauma and burns. Therefore, the skin is a common portal of entry of a wide variety of microbes and other foreign substances and is the site of many immune responses.
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