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Janeway CA Jr, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001.

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Immunobiology: The Immune System in Health and Disease. 5th edition.

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Chapter 2Innate Immunity

Throughout this book we will examine the individual mechanisms by which the adaptive immune response acts to protect the host from pathogenic infectious agents. In this chapter, however, we will examine the role of those innate, nonadaptive defenses that form early barriers to infectious disease. The microorganisms that are encountered daily in the life of a normal healthy individual only occasionally cause perceptible disease. Most are detected and destroyed within minutes or hours by defense mechanisms that do not require a prolonged period of induction because they do not rely on the clonal expansion of antigen-specific lymphocytes: these are the mechanisms of innate immunity.

The time course and different phases of an encounter with a new pathogen are summarized in Fig. 2.1. The innate immune mechanisms act immediately, and are followed by early induced responses, which can be activated by infection but do not generate lasting protective immunity. Only if an infectious organism can breach these early lines of defense will an adaptive immune response ensue, with the generation of antigen-specific effector cells that specifically target the pathogen, and memory cells that can prevent reinfection with the same microorganism. The power of adaptive immune responses is due to their antigen specificity, which we will be studying in the following chapters. However, they harness, and also depend upon, many of the effector mechanisms used by the innate immune system, which we will describe in this chapter.

Figure 2.1. The response to an initial infection occurs in three phases.

Figure 2.1

The response to an initial infection occurs in three phases. The effector mechanisms that remove the infectious agent (for example, phagocytes and complement) are similar or identical in each phase, but the first two phases rely on recognition of pathogens (more...)

Whereas the adaptive immune system uses a large repertoire of receptors encoded by rearranging genes to recognize a huge variety of antigens (see Section 1-10), innate immunity depends upon germline-encoded receptors to recognize features that are common to many pathogens. In fact, as we will see, the mechanisms of innate immunity discriminate very effectively between host cells and pathogen surfaces, and this ability to discriminate between self and nonself, and to recognize broad classes of pathogens, contributes to the induction of an appropriate adaptive immune response.

In the first part of the chapter we will consider the fixed defenses of the body: the epithelia that line the internal and external surfaces of the body, and the phagocytes that can engulf and digest invading microorganisms. As well as killing microorganisms, the activities of some of these phagocytes induce the next phase of the early response, and ultimately, if the infection is not cleared, the adaptive immune response. The second part of the chapter is devoted to a system of plasma proteins known as the complement system. This important element of innate immunity interacts with microorganisms to promote their removal by phagocytic cells. Next, we take a closer look at the receptors used by the immune system to recognize pathogens, and the last part of the chapter describes how the activation of phagocytic cells at the beginning of the innate immune response to infection leads to the induced or adaptive immune response.

Contents

  • The front line of host defense
  • The complement system and innate immunity
  • Receptors of the innate immune system
  • Induced innate responses to infection
  • Summary to Chapter 2
  • General references
  • Section references

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Garland Science.
Bookshelf ID: NBK10769

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