INTRODUCTION: The skin: The squamous epithelium of the skin is the first line of defense against microorganisms encountered in the outside world. As keratinized epithelial surface cells desquamate, the skin maintains its protective barrier by generating new epithelial cells beneath the surface. The skin is also bathed with oils and moisture from the sebaceous and sweat glands. These secretions contain fatty acids that inhibit bacterial growth. Poor vascular supply to the skin may result in skin breakdown and increased susceptibility to infection. For example, chronically debilitated or bedridden patients may suffer from presacral pressure ulcers as a result of constant compression of this dependent body area; these ulcers allow direct entry of skin and enteric bacteria into previously sterile sites and predispose patients to severe infections.
The mucus membranes: The mucous membranes also provide a physical barrier to microbial invasion. The mucous membranes of the mouth, pharynx, esophagus, and lower urinary tract are composed of several layers of epithelial cells, whereas those of the lower respiratory tract, the GI tract, and the upper urinary tract are delicate single layers of epithelial cells. These membranes are covered by a protective layer of mucus, which traps foreign particles and prevents them from accessing the lining of epithelial cells. Because mucus is hydrophilic, many substances produced by the body easily diffuse to the surface, including enzymes with antimicrobial activity such as lysozyme and peroxidase.
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