Figure 2.

Granuloma development during TB progression. The initial stages of TB infection involve inhalation of M. tuberculosis bacilli into the lung and phagocytosis by resident alveolar macrophages. Mtb elicits local inflammatory response resulting in the recruitment of monocytes and macrophages and other innate immune cells to the site of infection. The macrophages further differentiate into other specialized cells, such as epithelioid macrophages, foamy macrophages, and multi-nucleated giant cells. Upon induction of adaptive immunity, the granuloma gets a peripheral lymphocytic cuff mainly characterized by B and T cells. This is the balanced solid state in which many granulomas persist and restrict the bacilli at their center. However, as the disease progresses, increased necrotic breakdown of granuloma cells leads to the accumulation of caseum, which may result in cavitation of granulomas. Ultimately, as the granuloma cavitates and collapses into the lung, Mtb bacilli are released into the airway.