Pathogenesis of tuberculosis

When droplet nuclei are inhaled, most of the larger particles become lodged in the upper respiratory tract, where infection is unlikely to develop. However, smaller droplet nuclei containing the tubercle bacilli may reach the alveoli, where infection begins.

The tubercle bacilli that reach the alveoli are ingested by alveolar macrophages; the majority of these bacilli are destroyed or inhibited. A small number multiply intracellularly and are release when the macrophages die. These bacilli can spread through the lymphatic channels to regional lymph nodes and then through the bloodstream to more distant tissues and organs. The areas in which Tuberculosis
disease is most likely to develop are the apices of the lung, the kidneys, the brain, and bone. Extracellular bacilli attract macrophages from the bloodstream; this immune response kills most of the bacilli, leading to the formation of a granuloma. At this point the person has Tuberculosis infection,which can be detected by using the tuberculin skin test. It may take 2-10 weeks for the infected person to develop a positive reaction to the tuberculin skin test. Immune responses soon develop to kill the bacilli. Within 2 to 10 weeks after infection, the immune system is usually able to halt the multiplication of the tubercle bacilli, preventing further spread.

Persons who are infected with M. Tuberculosis but who do not have Tuberculosis disease cannot spread the infection to other people. Tuberculosis infection in a person who does not have Tuberculosis disease is not considered a case of Tuberculosis and is often referred to as latent Tuberculosis infection (LTBI).