ICSE Class 9 Answered

Tuberculosis (TB) disease can occur in pulmonary and extrapulmonary sites. TB disease most commonly affects the lungs and this is referred to as pulmonary TB. Extrapulmonary TB disease occurs in places other than the lungs, including the larynx, the lymph nodes, the pleura, the brain, the kidneys, or the bones and joints.

Infection by Mycobacterium tuberculosis occurs when a person inhales droplet nuclei containing tubercle bacilli that reach the alveoli of the lungs. These tubercle bacilli are ingested by alveolar macrophages, wherein majority of these bacilli are destroyed or inhibited while a small number of bacilli may multiply intracellularly. They are released when the macrophages die. If alive, these bacilli may spread by the way of lymphatic channels or through the bloodstream to more distant tissues and organs including the areas of the body in which TB disease is most likely to develop; regional lymph nodes, apex of the lung, kidneys, brain, and bone.