Tuberculosis (TB) is an infectious disease, primarily caused by the bacterium Mycobacterium tuberculosis, that remains a significant global health concern. It predominantly affects the lungs, making it a major respiratory illness worldwide.
Initial Invasion and Immune Response
Tuberculosis infection begins when an individual inhales tiny airborne droplets containing Mycobacterium tuberculosis bacteria. Once these bacteria enter the lungs, they are encountered by alveolar macrophages, the body’s initial line of defense. These immune cells attempt to engulf and destroy the invading bacilli.
However, Mycobacterium tuberculosis has evolved mechanisms to survive and multiply within macrophages. This infection triggers a localized inflammatory response, attracting immune cells to the site. These cells aggregate to form a granuloma, which contains the infection and prevents its spread. In many cases, the immune system successfully walls off the bacteria within these granulomas, leading to a latent TB infection where the bacteria remain alive but inactive, causing no symptoms and preventing transmission.
Progression and Lung Damage
While latent infection can persist for a lifetime without causing illness, a weakened immune system can lead to the reactivation of dormant bacteria or progression into active tuberculosis. The contained bacteria within granulomas then begin to multiply. Granulomas undergo caseous necrosis, a process involving the death of tissue within the granuloma, resulting in a soft, crumbly, cheese-like material.
As caseous necrosis progresses, it destroys functional lung tissue, known as parenchyma, leading to the formation of cavities within the lungs. These cavities develop when necrotic material liquefies and is expelled, often through coughing, leaving empty pockets. This destruction impairs the lung’s ability to exchange gases efficiently, reducing oxygen uptake and carbon dioxide removal. The healing process following this damage often involves fibrosis, where normal lung tissue is replaced by scar tissue, diminishing lung function and altering lung architecture.
Symptoms and Contagion
Extensive lung damage caused by active tuberculosis leads to characteristic symptoms. As the infection destroys lung tissue and forms cavities, compromised pulmonary function results in a persistent cough, which can last longer than two weeks. This cough may produce sputum or, in severe cases, blood. Inflammation and tissue destruction within the lungs can also cause chest pain and shortness of breath.
The presence of active Mycobacterium tuberculosis bacteria within the airways, often due to the breakdown of lung tissue, makes an infected individual contagious. When a person with active pulmonary TB coughs or sneezes, they release tiny airborne droplets containing the bacteria. These droplets can remain in the air for several hours, allowing others to inhale them and become infected. This mechanism of airborne transmission poses a public health risk, particularly in crowded or poorly ventilated environments.
Long-Term Consequences for Lung Health
Even after successful treatment, tuberculosis can leave a lasting impact on lung health. Extensive tissue destruction and subsequent healing often result in permanent lung scarring, or fibrosis, which can reduce overall lung capacity. This scarring can lead to restrictive lung disease, limiting the amount of air the lungs can hold. Some individuals may also develop bronchiectasis, a condition characterized by permanent widening and damage of the airways, predisposing them to recurrent respiratory infections and chronic cough.
Post-TB lung disease can also manifest as chronic obstructive pulmonary disease (COPD) or other forms of airflow obstruction, making breathing difficult and less efficient. These long-term consequences can significantly affect a person’s quality of life, impacting their ability to perform daily activities and exercise. The extent of these effects often depends on the severity of the initial infection and how promptly and effectively treatment was administered.