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English: Schematic diagram of latent tuberculosis infection and its mechanism. Latent tuberculosis infection (LTBI) is a balance between immunity of host and invasiveness of M. tuberculosis; any tilt would upset the balance (A). The M. tuberculosis excreted from patients with active TB is inhaled by healthy people through airborne transmission and recognized and phagocytized by antigen-presenting cells such as macrophages, neutrophils, natural killer cells, and B lymphocytes (B). Then, neutrophils release cytokines to activate and recruit T lymphocytes to gather at the place where M. tuberculosis invades to form granulomas (B). If the host immunity is strong, M. tuberculosis will be cleared by immune cells and the host recovers; If the host immunity is weak, M. tuberculosis will reproduce in the granulomatous tissue and breaks through the granulomatous restriction to cause active TB; If the immunity of host and invasiveness of M. tuberculosis is balanced, the host will be latently infected (A,B).