Although antimicrobial host defense reactions are numerous and varied, there are several important general features of immunity to microbes.
• Defense against microbes is mediated by the effector mechanisms of innate and adaptive immunity. The innate immune system provides early defense, and the adaptive immune system provides a more sustained and stronger response. Many pathogenic microbes have evolved to resist innate immunity, and protection against such infections is critically dependent on adaptive immune responses. Adaptive immune responses to microbes are more specific than innate responses. Adaptive responses induce large numbers of effector cells that function to eliminate the microbes and also generate memory cells that protect the individual from repeated infections.
• The immune system responds in distinct and specialized ways to different types of microbes to most effectively combat these infectious agents. Because microbes differ greatly in patterns of host colonization and invasion, their elimination requires diverse effector systems. The specialization of adaptive immunity allows the host to respond optimally to different types of microbes. The generation of TH1, TH2, and TH17 subsets of effector CD4+ T cells and the production of different isotypes of antibodies are excellent examples of the specialization of adaptive immunity. Both have been described in earlier chapters; their importance in defense against different types of microbes is mentioned in this chapter.
• The survival and pathogenicity of microbes in a host are critically influenced by the ability of the microbes to evade or resist the effector mechanisms of immunity. Infectious microbes and the immune system have coevolved and are engaged in a constant struggle for survival. The balance between host immune responses and microbial strategies for resisting immunity often determines the outcome of infections. As we shall see later in this chapter, microorganisms have developed a variety of mechanisms for surviving in the face of powerful immunologic defenses.
• Many microbes establish latent, or persistent, infections in which the immune response controls but does not eliminate the microbe and the microbe survives without propagating the infection. Latency is a feature of infections by several viruses, especially DNA viruses of the herpesvirus and poxvirus families, and some intracellular bacteria. In latent viral infections, the viral DNA may be integrated into the DNA of infected cells, but no infectious virus is produced. In persistent bacterial infections such as tuberculosis, the bacteria may survive within the endosomal vesicles of infected cells. In all these situations, if the host's immune system becomes defective for any reason (such as cancer or therapy for cancer, immunosuppression to treat transplant rejection, or HIV infection), the latent microbe may be reactivated, resulting in an infection that causes significant clinical problems.
• In many infections, tissue injury and disease may be caused by the host response to the microbe and its products rather than by the microbe itself. Immunity, like many other defense mechanisms, is necessary for host survival but also has the potential for causing injury to the host.
This chapter considers the main features of immunity to five major categories of pathogenic microorganisms: extracellular bacteria, intracellular bacteria, fungi, viruses, and protozoan and multicellular parasites (Table 15-1). Our discussion of the immune responses to these microbes illustrates the diversity of antimicrobial immunity and the physiologic significance of the effector functions of lymphocytes discussed in earlier chapters.
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