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FUNCTION OF THE IMMUNE SYSTEM

The main function of the immune system is to prevent or limit infections, fungi, and parasites, such as protozoa and worms. The first line of defense against microorganisms is the intact skin and mucous membranes. If microorganisms breach this line and enter the body, then the innate arm of the immune system (second line of defense) is available to destroy the invaders. Because the components of the innate arm (Table 57–1) are preformed and fully active, they can function immediately upon entry of the microorganisms. The ability of the innate arm to kill microorganisms is not specific. For example, a neutrophil can ingest and destroy many different kinds of bacteria.

TABLE 57–1Main Components of Innate and Adaptive Immunity That Contribute to Humoral (Antibody-Mediated) Immunity and Cell-Mediated Immunity

Highly specific protection is provided by the adaptive (acquired) arm of the immune system (third line of defense), but it takes several days for this arm to become fully functional. The two components of the adaptive arm are cell-mediated immunity and antibody-mediated (humoral) immunity. An overview of the functions and interactions between many of the important members of the innate and adaptive arms of the immune response is provided in Figure 57–1. (The features of the innate and the adaptive arms of the immune system are contrasted in Table 57–2.)

FIGURE 57–1

Introduction to the interactions and functions of the major components of the immune system. Left: Antibody-mediated (humoral) immunity. This is our main defense against extracellular, encapsulated, pyogenic bacteria such as staphylococci and streptococci. Antibodies also neutralize toxins, such as tetanus toxin, as well as viruses, such as hepatitis B virus. Right: Cell-mediated immunity. There are two distinct components. (1) Helper T cells and macrophages are our main defense against intracellular bacteria, such as M. tuberculosis, and fungi, such as Histoplasma capsulatum. (2) Cytotoxic T cells are an important defense against viruses and act by destroying virus-infected cells. (IL-4 and IL-5 are interleukin-4 and interleukin-5, respectively.)

TABLE 57–2Important Features of Innate and Adaptive Immunity

The cell-mediated arm consists primarily of T lymphocytes (e.g., helper T ...

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