Pathogenesis

Tuberculosis is usually contracted by inhaling airborne organisms from a person with tuberculosis. In the lungs, the bacteria are taken up by pulmonary macrophages, where they resist destruction and multiply. Within the macrophages, they are carried to nearby lymph nodes. At this stage, there is little inflammatory response to the infection. The organisms continue to multiply, lyse the macrophages, and spread throughout the body. After about 2 weeks, delayed hypersensitivity to the tubercle bacilli develops. An intense reaction then occurs at sites where the bacilli have lodged. Macrophages, now activated, collect around the bacteria, and some macrophages fuse together to form large multinucleated giant cells. Lymphocytes and macrophages then collect around these multinucleated cells and wall off the infected area from the surrounding tissue. The localized collection of

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Figure 23.17 Mycobacterium tuberculosis as Seen in Sputum from an Individual with Tuberculosis The sputum has been "digested," meaning that it has been treated with strong alkali to kill the other bacteria that are invariably present.

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Figure 23.17 Mycobacterium tuberculosis as Seen in Sputum from an Individual with Tuberculosis The sputum has been "digested," meaning that it has been treated with strong alkali to kill the other bacteria that are invariably present.

inflammatory cells is called a granuloma (figure 23.18), which is the characteristic response of the body to microorganisms and other foreign substances that resist digestion and removal. The granulomas of tuberculosis are called tubercles. Although they may not kill the mycobacteria, they generally halt growth of the organisms, and usually no significant illness develops. Commonly, M. tuberculosis remains alive in the tubercles for many years. ■ delayed hypersensitivity, p. 449

Sometimes the mycobacteria are not contained by the inflammatory response. Lysis of activated macrophages attacking M. tuberculosis releases their enzymes into the infected tissue. The result is death of tissue, with the formation of a cheesy material by a process called caseous necrosis. If this process involves a bronchus, the dead material may discharge into the airways, causing a large lung defect called a cavity and spread of the bacteria to other parts of the lung. Lung cavities characteristically persist, slowly enlarging for months or years and shedding tubercle bacilli into the bronchi. Coughing and spitting transmit the organisms to other people. In areas of caseous necrosis, M. tuberculosis often remains alive, although multiplication ceases. Wherever living organisms persist, they can resume growing if the person's immunity becomes impaired by stress, advanced age, or AIDS. Disease resulting from renewed growth of the organisms is called reactivation tuberculosis.

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