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The main function of the immune system is to prevent or limit infections due to viruses, bacteria, fungi, protozoa, and worms. The first line of defense against microorganisms is the barrier formed by intact skin and mucous membranes. If microorganisms breach this line and enter the body, then a second line of defense is available to rapidly detect foreign material and destroy any harmful agents. These components of the immune system are preformed and encoded in the genome, and therefore, this arm of host defense is called innate immunity (Table 57–1). Innate immunity works immediately upon the first encounter with a microorganism. The innate arm is nonspecific in that it can recognize patterns shared among many microorganisms (described in more detail in Chapter 58). For example, a neutrophil can sense, ingest, and destroy many different kinds of bacteria by exploiting features common among bacterial cells.

TABLE 57–1Important Features of Innate and Adaptive Immunity

Some microbes can mutate to resist the tactics of innate immunity. For these microbes, there is a more targeted immune protection that is specific for individual infectious agents, which is provided by the adaptive (acquired) arm of the immune system (often considered the third line of defense). The adaptive arm takes days to become fully functional, but once engaged, it remembers an infectious agent and responds more quickly to repeat encounters. For example, after receiving the first dose of the pneumococcal vaccine, it takes 7 to 10 days to produce protective levels of antibodies, but when you get a booster, this takes only 2 to 3 days. Table 57–1 provides a summary of the features of innate and adaptive immunity.

Historically, the immune system has also been organized into a cell-mediated arm (primarily orchestrated by T lymphocytes) and a humoral arm (freely circulating factors, such as antibodies, whose activity is enhanced by complement proteins). This chapter will introduce the central players of the innate and adaptive arms of the immune system, and subsequent chapters will discuss in more detail how they cooperate during normal immune responses and how their failure can cause disease.


Our immune host defenses can be divided into two major categories: innate (natural) and adaptive (acquired) (see Table 57–1).

1. Innate Immunity

Properties of Innate Immunity

At the time of birth, you ...

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