The innate immune system blocks the entry of microbes and eliminates or limits the growth of many microbes that are able to colonize tissues. The main sites of interaction between individuals and their environment— the skin and gastrointestinal and respiratory tracts— are lined by continuous epithelia, which serve as barriers to prevent the entry of microbes from the external environment. If microbes successfully breach the epithelial barriers, they encounter the cells of innate immunity. The cellular innate immune response to microbes consists of two main types of reactions—inflammation and antiviral defense. Inflammation is the process of recruitment of leukocytes and plasma proteins from the blood, their accumulation in tissues, and their activation to destroy the microbes. Many of these reactions involve cytokines, which are produced by dendritic cells, macrophages, and other types of cells during innate immune reactions. The major leukocytes that are recruited in inflammation are the phagocytes, neutrophils (which have short life spans in tissues), and monocytes (which develop into tissue macrophages). These phagocytes express on their surfaces receptors that bind and ingest microbes and other receptors that recognize different microbial molecules and activate the cells. On engagement of these receptors, the phagocytes produce reactive oxygen and nitrogen species and lysosomal enzymes, which destroy the microbes that have been ingested. Resident macrophages in the tissues serve much the same functions. Antiviral defense consists of a cytokine-mediated reaction in which cells acquire resistance to viral infection and killing of virus-infected cells by NK cells.
Microbes that are able to withstand these defense reactions in the tissues may enter the blood, where they are recognized by the circulating proteins of innate immunity. Among the most important plasma proteins of innate immunity are the components of the alternative pathway of the complement system. When this pathway is activated by microbial surfaces, proteolytic cleavage products are generated that mediate inflammatory responses, coat the microbes for enhanced phagocytosis, and directly lyse microbes. (As we shall discuss later, complement can also be activated by antibodies—called the classical pathway, for historical reasons—with the same functional consequences.) Many of the circulating proteins enter sites of infection during inflammatory reactions and thus help combat microbes in extravascular tissues.
The reactions of innate immunity are effective at controlling and even eradicating infections. However, a hallmark of many pathogenic microbes is that they have evolved to resist innate immunity. Defense against these pathogens requires the more powerful and specialized mechanisms of adaptive immunity, which prevents them from invading and replicating in the cells and tissues of the host.
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