Tuberculosis

Tuberculosis (TB) remains a significant global health concern, primarily targeting the lungs and spreading through airborne transmission. Infection begins when aerosolized droplet nuclei, expelled by an individual with active TB, are inhaled by another person. These microscopic particles carry Mycobacterium tuberculosis, the causative agent of TB. Upon reaching the alveoli, the bacilli are engulfed by alveolar macrophages. However, due to their specialized lipid-rich cell wall, these pathogens resist destruction and survive within the phagocytes.

Intracellular Survival and Dissemination

Following phagocytosis, infected macrophages traverse the alveolar epithelium, disseminating M. tuberculosis into the surrounding lung tissue and regional lymph nodes. The persistence of the bacteria is primarily attributed to the presence of cord factor (trehalose 6,6’-dimycolate), a glycolipid on the bacterial cell wall. This virulence factor induces the formation of serpentine cords by promoting bacterial aggregation, thereby evading effective immune clearance. These cords are inefficiently phagocytosed by interstitial macrophages, leading to the recruitment of additional immune cells, including lymphocytes, to the site of infection.

Granuloma Formation and Latency

The host immune response culminates in the formation of granulomas—organized structures composed of macrophages, epithelioid cells, and T lymphocytes that encase the infected cells. Within these granulomas, the bacteria can remain viable yet dormant for extended periods, establishing latent TB. This phase is clinically asymptomatic and non-contagious but poses a risk for future reactivation.

Reactivation and Transmission

Reactivation of latent tuberculosis can happen under immunocompromising conditions such as malnutrition, advanced age, or co-infection with HIV. In older individuals, reduced T-cell production due to thymic shrinkage weakens immune defense. In malnourished individuals, a low white blood cell count further reduces the body’s ability to control infection. Because of this, the immune system may no longer maintain the integrity of granulomas, allowing Mycobacterium tuberculosis to escape into the pulmonary environment and multiply rapidly. This leads to active tuberculosis, marked by inflammation, tissue damage, and symptoms such as a persistent cough. Coughing releases infectious droplets into the air, helping the disease spread easily.