Tuesday, 17 February 2015

PATHOGENESIS OF TUBERCULOSIS


Tuberculosis is an air borne infection !!

1) Once the mycobacteria enters the respiratory tract of a person it reaches the alveoli and is engulfed by the alveolar macrophages

2) Mycobacteria resists killing by macrophages , so macrophage containing bacteria enters the interstitium , now the infected macrophage will produce interleukin - 12 

3) Interleukin -12 will recruit T-Helper -1 cells at that site .

4) T-helper -1 cells after interacting with infected macrophage will start producing INTERFERON-gamma .

5)INTERFERON-gamma  will start recruiting more macrophages at that site . but macrophages cannot kill the bacilli .


6) So macrophages will transform themselves into flattened cells . These flattened macrophages is called as EPITHELIOID CELL .

7)These epithelioid cells will produce TUMOR NECROSING FACTOR (TNF) . TNF will recruit more macrophages to the infected site and convert them to epithelioid cell .

8)These epithelioid cells will form a wall around the affected site . MOTIVE of forming a wall is because these epithelioid cells (macrophages) cannot kill the mycobacteria , so by forming a wall around the site , they can at least prevent the spread of infection to other sites of the lungs and other organs .  

9)Some epithelioid cells will clump together and form a giant cell . this giant cell is called as langhan's giant cell . This giant cell have an characteristic HORSESHOE arrangement of nucleus .

10) Mycobacteria can multiply inside the macrophage and destroy the macrophages . Thus there is an characteristic necrosis seen at the infected site of tuberculosis , this is called as CASEOUS NECROSIS

11) Caseous necrosis in the middle surrounded by epithelioid cells and t cells is called as GRANULOMA OF TUBERCULOSIS.





















 We saw about basic pathogenesis of TB. 
 Next i will explain about PRIMARY AND SECONDARY  TUBERCULOSIS.


 PRIMARY TUBERCULOSIS : A PREVIOUSLY UNINFECTED PERSON WHEN GETS INFECTED BY TUBERCULOSIS FOR THE FIRST TIME , IT IS CALLED AS PRIMARY TUBERCULOSIS .

Primary site of granuloma formation is called as GHON'S FOCUS . More often a infected macrophage reaches the hilar lymph node and forms a granuloma there . 
Ghon's focus + Involved hilar lymph node together is called as GHON'S COMPLEX.

1) 70% of times , when a patient get affected by primary TB, he is symptomless (latent TB ) because the infection is well contained by the epithelioid cell . 

2) Only 20% shows some symptoms .

3) Sometimes primary tuberculosis can progress to a fatal condition . This is called as MILIARY TUBERCULOSIS .It is commonly seen in immunodeficient or immune suppressed patients . In miliary tuberculosis there is multiple granuloma formed in the lungs and there is involvement of extra pulmonary sites ( kidneys , brain , bones ) also. 

Later in life Will develop tuberculosis due to reactivation of the dormant bacteria . This is called as SECONDARY TUBERCULOSIS .
In Secondary tuberculosis the symptoms are more severe than the primary Tb . 
Typical brown coloured sputum producing cough is seen in secondary tuberculosis . 
Even secondary tuberculosis can progress to miliary Tb .











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