Giant multinucleated cells are seen in the early stages of active infection with the acid-fast bacterium Mycobacterium tuberculosis, the causative agent of the disease tuberculosis.
Following inhalation of the M. tuberculosis bacterium and its attachment to the lumen of the alveolus, immune cells known as macrophages identify the bacterium as "foreign" and attempt to eliminate it by phagocytosis. During this process, the entire bacterium is enveloped by the macrophage and stored temporarily in a membrane-bound vesicle called a phagosome. The phagosome then combines with a lysosome to create a phagolysosome. In the phagolysosome, the cell attempts to use reactive oxygen species and acid to kill the bacterium. However, M. tuberculosis has a thick, waxy mycolic acid capsule that protects it from these toxic substances. M. tuberculosis actually reproduces inside the macrophage and will eventually kill the immune cell. Other macrophages attack the infected macrophage, fusing together to form a giant multinucleated cell in the alveolar lumen.
Transformation into tubercules 
The giant multinucleated cell eventually becomes necrotic and dies. However, the M. tuberculosis bacterium inside is still viable. When the periphery of the giant multinucleated cell becomes calcified, a tubercule (also known as a granuloma) is formed, which remains in the lungs and may eventually need to be surgically excised. The giant multinucleated cell can also liquefy and spread the bacterial infection to the blood or other tissues.
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