Porth's Essentials of Pathophysiology, 4e

549

Respiratory Tract Infections, Neoplasms, and Childhood Disorders

C h a p t e r 2 2

Inhalation of tubercle bacillus

Secondary tuberculosis

Primary tuberculosis

Development of cell-mediated immunity

Cell-mediated hypersensitivity response

Reinfection

Granulomatous inflammatory response

Positive skin test

Progressive or disseminated tuberculosis

Ghon complex

node granulomas is called a Ghon complex (Fig. 22-6). The Ghon complex eventually heals, undergoing shrink- age, fibrous scarring, and calcification, the last of these being visible radiographically. However, small num- bers of organisms may remain viable for years. Later, if immune mechanisms decline or fail, latent tuberculosis infection has the potential to develop into secondary tuberculosis. PrimaryTuberculosis Primary tuberculosis is a form of the disease that devel- ops in previously unexposed, and therefore unsensi- tized, persons. 2,13,32,33 It typically is initiated as a result of inhaling droplet nuclei that contain the tubercle bacil- lus (Fig. 22-7). Most people with primary tuberculosis are asymptomatic and go on to develop latent tuber- culosis infection in which T lymphocytes and macro- phages surround the organism in granulomas that limit their spread. Individuals with latent tuberculosis do not have active disease and cannot transmit the organism to others. 34 In approximately 5% of newly infected people, the immune response is inadequate; these people go on to develop progressive primary tuberculosis with continued destruction of pulmonary tissue and spread to multiple sites within the lung. 2,32,33 This usually occurs in young children, whose immune systems are immature, or in adults with HIV infection or other immunodeficiency FIGURE 22-6. Primary tuberculosis. A healed Ghon complex is represented by a subpleural nodule and involved hilar lymph nodes. (From Beasley MB,TravisWD, Rubin E.The respiratory system. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins; 2012:550.)

Healed dormant lesion

Reactivated tuberculosis

FIGURE 22-7. Pathogenesis of tuberculosis infection.

disorders. Sometimes the onset of symptoms is abrupt, with high fever, pleuritis, and lymphadenitis. As the dis- ease spreads, the organism gains access to the sputum, allowing the person to infect others. In rare instances, tuberculosis may erode into a blood vessel, giving rise to hematogenic dissemination. Miliary tuberculosis describes minute lesions, resembling millet seeds, resulting from this type of dissemination that can involve almost any organ, particularly the brain, menin- ges, liver, kidney, and bone marrow. SecondaryTuberculosis Secondary tuberculosis represents either reinfection from inhaled droplet nuclei or reactivation of a previ- ously healed primary lesion 2 (see Fig. 22-7). It often occurs in situations of impaired body defense mecha- nisms. The partial immunity that follows primary tuber- culosis normally affords protection against reinfection and helps localize the disease should reactivation occur. After the development of hypersensitivity, the infection becomes quiescent in the majority of patients. Cavities are formed as a result of the immune response walling off the infection. These cavities may coalesce to a size of up to 10 to 15 cm in diameter (Fig. 22-8). In vulner- able patients such as the very young or immunocom- promised adults, the primary infection can progress into