Pathogenesis of TB

Tuberculosis (TB) is an infectious, chronic, granulomatous disease caused by Mycobacterium tuberculosis. It usually involves the lungs but may affect any organ or tissue in the body.

The pathogenesis of TB in a previously unexposed immunocompetent individual is centred on the development of cell-mediated immunity. This confers resistance to the organism and results in the development of tissue hypersensitivity to tubercular antigens.

The pathologic features of TB, such as caseating granulomas and cavitation, result from the destructive tissue hypersensitivity that is part and parcel of the host immune response.

The sequence of events from inhalation of the infectious droplets to the containment of the primary focus of infection is as follows:

Figure: Pathogenesis of Tuberculosis

In many individuals, the stage 5 response (from above) halts the infection before significant tissue destruction or illness occur (Latent TB Infection). In other individuals with immune deficits due to age or immunosuppression, the infection progresses to stage 5, and the ongoing immune response results in caseation necrosis (Active TB Disease). The figure above provides more details on the progression of TB disease.

Resources

• Robbins Basic Pathology, 10^th Edition.
• Transmission and Pathogenesis of Tuberculosis, CDC.

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