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FUNCTION OF THE IMMUNE SYSTEM
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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 defense, then a second line of defense can rapidly detect foreign material and destroy harmful agents. These components of the immune system are active even before an infectious exposure, 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 many microorganisms share (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.
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Some microbes can mutate to resist the tactics of innate immunity. For these microbes, there is a more targeted defense 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.
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The immune system has a cell-mediated arm (orchestrated by T lymphocytes) and a humoral arm (circulating factors, such as antibodies and complement proteins). The combined effects of immune cells (e.g., T cells, macrophages) and proteins (e.g., antibodies, complement) produce inflammation (see Chapter 8). This chapter will introduce the innate and adaptive immune system, and subsequent chapters will discuss how they cooperate during normal immune responses and how their failure can cause disease.
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INNATE & ADAPTIVE IMMUNITY
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At the time of birth, you already have a powerful arsenal of immune defenses at work. These defenses exist, fully encoded in your genes, prior to exposure to any microbes, and because of this, they are called innate. Innate immunity is nonspecific and includes barriers ...