Progressive primary TB infection (Part 3)
Welcome to the 3rd part of the multi-part series on Tuberculosis, abbreviated as TB.
In Part 2, we studied that in around 90% of the individuals, either TB remains in the latent state or is eliminated from the body.
In other words, 90% of the individuals do not develop TB disease.
However, in the remaining 10% of individuals, the consequences of TB infection are different. Among them, in the case of 5% of individuals, the primary infection is progressive, i.e., the disease progresses to pulmonary or extra pulmonary tuberculosis. The reason behind the progression of the disease can either be a large number of TB bacilli in the body or the weakened immune system because of which macrophages and T cells are unable to contain bacteria. As a result, the TB bacilli continue to multiply exponentially.
We have already discussed in Part 2 that massive activation of macrophages within granuloma often results in the release of lytic enzymes. These enzymes result in caseous lesions.
And if the large concentrations of Mtb are present within granuloma, then a large number of CD4 T cells will be activated and will ultimately lead to the activation of large numbers of macrophages. Thus the lytic enzymes released by activated macrophages will be in high concentration, which will cause the necrotic caseous lesions to rupture.
And we have discussed previously that during the primary infection, these caseous lesions help to contain bacteria. The rupturing of caseous lesions literally creates a hole in the lung where the tissue has broken down completely. This hole allows the bacilli to disseminate outside granuloma. This dissemination of the bacilli outside the granuloma ends up causing pulmonary tuberculosis i.e., TB of the lungs.
In some cases, the infection may not be limited to the lungs only and may go even beyond that. The infection may disseminate widely and may spread through the blood and lymphatic vessels to many organs of the body like the liver, bone, kidney, and even to the brain and many other organs. TB infection in other parts of the body than the lungs is called extrapulmonary TB. Extrapulmonary TB will be discussed in detail in Part 6.
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