Droplet inhalation is the primary mode of infection
Infection by drinking milk contaminated with Mycobacterium bovis is now rare in developed nations
M. avium-intracellulare, are much less virulent than M. tuberculosis and rarely cause disease in immunocompetent individuals
The bacilli are obligate aerobes whose slow growth is retarded by a pH lower than 6.5, hence not found in the center of granuloma
caseating granulomas and cavitation, are the result of the destructive tissue hypersensitivity that is part and parcel of the host immune response
Pulmonary alveolar macrophages engulf the bacteria
endosomal manipulation by the bacteria in the macrophaghe impairs phagolysosome formation, allowing unhindered mycobacterial proliferation
bacillary proliferation within the pulmonary alveolar macrophages and airspaces, with resulting bacteremia and seeding of multiple sites
development of cell-mediated immunity occurs approximately 3 weeks after exposure
Macrophages carry bacilli to the draining lymph nodes and are presented in a MHCII by dendritic cell/macrophages( APCs) to CD4+ T cells.
IFN-γ released by the CD4+ T cells of the TH1 subset is crucial in activating macrophages.