Animation of a macrophage white blood cell infected with a Mycobacterium tuberculosis bacterium (red). Macrophages are cells of the body's immune system that phagocytose and destroy pathogens, dead cells and cellular debris. TB infection begins when the mycobacteria reach the pulmonary alveoli in the lungs, where they invade and replicate within the endosomes of alveolar macrophages. M. tuberculosis uses a number of strategies to avoid being killed by the macrophage. It inhibits the phagosome that it is encapsulated in from maturing. This prevents fusing with lysosomes, which contain enzymes that digest the bacteria.
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