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The human immune system is comprised of two distinct functional parts: (1) innate and (2) adaptive. These two components have different types of recognition receptors and differ in the speed in which they respond to a potential threat to the host (Fig. 10-1). Cells of the innate immune system, including macrophages and dendritic cells (DCs), use pattern recognition receptors encoded directly by the germ line DNA, respond to biochemical structures commonly shared by a variety of different pathogens, and elicit a rapid response against these pathogens, although no lasting immunity is generated. In contrast, cells of the adaptive immune system, T and B lymphocytes, bear specific antigen receptors encoded by rearranged genes, and in comparison to the innate response, adaptive immunity develops more slowly. A unique feature of the adaptive immune response is its ability to generate and retain memory; thus, it has the capability of providing a more rapid response in the event of subsequent immunologic challenge. Although the innate and adaptive immune responses are distinct, they interact and can each influence the magnitude and type of their counterpart. Together, the innate and adaptive immune systems act in synergy to defend the host against infection and cancer. This chapter describes the roles of the innate and adaptive immune response in generating host defense mechanisms in skin.
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Immune mechanisms that are used by the host to immediately defend itself are referred to as innate immunity. These include physical barriers such as the skin and mucosal epithelium; soluble factors such as complement, antimicrobial peptides, chemokines, and cytokines; and cells, including monocytes/macrophages, DCs, natural killer cells (NK cells), and polymorphonuclear leukocytes (PMNs) (Fig. 10-2).
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