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Pathophysiology of TB

Pathophysiology of TB. The Pathogens. TB is mainly caused by Mycobacterium tuberculosis . It can occasionally be caused by M. bovis or M. africanum . M. tuberculosis divides every 15-20 hours.

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Pathophysiology of TB

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  1. Pathophysiology of TB

  2. The Pathogens • TB is mainly caused by Mycobacterium tuberculosis. • It can occasionally be caused by M. bovisor M. africanum. • M. tuberculosis divides every 15-20 hours. • It is has a thick cell wall rich in lipids which prevents it taking up most stains and helps it resist digestion in macrophages. • It is an aerobe & an acid fast bacillus.

  3. Infection & Dormancy • M. tuberculosis is spread in aerosols released by coughing/sneezing. It needs to be inhaled for infection to occur. • Once inhaled, the bacteria reach the alveoli and are phagocytosed by the alveolar macrophages. Their lipid coating and ability to inhibit phagosome-lysosome fusion enables them to avoid digestion. • This primary infection site is called a Ghon focus and is usually in the lower part of the upper lobe or the upper part of the lower lobe. • The bacteria soon reach the lymph nodes at the hilum of the lung. The ghon focus and the infected node constitute a Ghon complex. These are visible on X ray.

  4. Infection & Dormancy ctd. • The cell mediated immune reaction causes the formation of granulomas. • These are composed of numerous leukocytes surrounding a core of infected macrophages. • Most of the bacteria are destroyed but some enter a dormant state and survive by slowing down their metabolism. • Cells in the centre of the granulomas undergo necrosis. The resulting dead matter looks pale and cheesy and is called caseous necrosis. • Some granulomas undergo calcification and can be seen on X-rays after the disease ceases to be active.

  5. Reactivation • The primary infection may not be self limiting if the host is very young/old or immunocompromised. • When the immune system is compromised in someone with latent TB (eg- HIV, diabetes, steroids) the M. tuberculosis can reactivate and cause secondary TB. • Unlike the primary infection this is not self limiting. • The bacteria can spread to many parts of the body and cause serious illness- eg: GIT, brain, liver

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