Pathogenesis Of Tuberculosis

Describe the pathogenesis of tuberculosis.
Answer:

Pathogenesis of Tuberculosis

The interaction of bacilli and the host begins when droplet nuclei from infectious patients are inhaled.
The majority of the bacilli are trapped and exhaled by ciliary action and a fraction less than 10% enters the alveoli.
In the initial stage of host-bacterial interaction, either the host’s macrophages control the multiplication of the bacteria or the bacteria grow and kill the macrophages.
Non-activated monocytes attacked from the bloodstream to the site by various chemotactic factors ingest the bacilli released from the lysed macrophages.
Initial stages are asymptomatic; about 2-4 weeks after infection tissue damaging and macrophage activating responses develop.
With the development of specific immunity and accumulation of a large number of activated macrophages at the site of the primary lesion, granulomatous reactions or tubercles are formed.

Pathogenesis of tuberculosis

The hard tubercle consists of epithelioid cells, Langhans giant cells, plasma cells, and fibroblasts. These lesions develop when host resistance is high.
Due to cell-mediated immunity in the majority of individuals, local macrophages are activated and lymphokines are released, which neutralize the bacilli and prevent further tissue destruction.
The central part of the lesion contains caseous, soft, and cheesy necrotic material (caseous necrosis). This necrotic material may undergo calcification at a later stage called
Ranne complex, in the lung parenchyma and hilar lymph nodes in few cases.
Caseous necrotic material undergoes liquefaction and discharges into the lungs leading to the formation of a cavity. Spontaneous healing of the cavity occurs either by fibrosis or collapse.
Calcification of the cavities may occur in which bacteria persist.
In the early stages, the spread of infection is mainly by macrophages to lymph nodes, other tissues and organs: However, in children with poor immunity hematogenous spread results in fatal miliary TB or tuberculous meningitis.

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