3.7 : Pulmonary Tuberculosis II

Tuberculosis, or TB, is a bacterial infectious disease caused by Mycobacterium tuberculosis. While its primary impact is on the lungs, leading to pulmonary tuberculosis, it can also affect various other organs, a condition referred to as extrapulmonary tuberculosis.

Here is a detailed explanation of its pathophysiology:

Transmission: The process begins when a person inhales droplet nuclei containing M. tuberculosis. These are typically released into the air when an individual with pulmonary or laryngeal TB coughs, sneezes, or speaks.

Primary Infection: In this initial phase, inhaled bacteria reach the alveoli in the lungs, encountering alveolar macrophages. The macrophages engulf the bacteria, but the bacteria survive and replicate within instead of being destroyed.

Cell-Mediated Immune Response: Approximately 2-6 weeks post-infection, the body mounts a cell-mediated immune response. T cells recognize mycobacterial antigens presented by the infected macrophages. It triggers cytokine release, activating other immune cells and processes.

Granuloma Formation: Infected macrophages and T cells cluster together to form a granuloma, an immune response that aims to contain the spread of the bacteria. Within the granuloma, the bacteria can remain dormant (latent TB infection) for many years, causing no symptoms and not being contagious.

Progression to Active Disease: In about 5-10% of individuals with latent TB infection, it is asymptomatic and not contagious. In a latent TB infection, the pathogen is not always dormant, as the bacteria can still be alive but controlled by the immune system, and can still cause active TB disease. This commonly occurs when the immune system is weakened, as observed in cases of HIV infection. The bacteria multiply and spread, leading to tissue damage and the symptoms of TB.

Caseation and Cavitation: The caseation process can occur within the granuloma, where the center becomes necrotic and cheese-like, often worsening symptoms. If this caseous material erodes into a bronchus, it forms a cavity, which allows the bacteria to multiply rapidly.

Dissemination: From the lungs, the bacteria can spread through the bloodstream (miliary TB) or the lymphatic system to other organs, causing extrapulmonary TB.

It is worth noting that not everyone who contracts TB will develop an active disease. Many individuals have latent TB infection, which is asymptomatic and non-infectious.

There is a potential risk for them to develop active tuberculosis in the future, particularly if their immune system becomes compromised.