Since inhaled droplet nuclei (containing M. tuberculosis) are so small, they avoid the mucociliary defenses of the bronchi and lodge in the terminal alveoli of the lungs where they are engulfed by the alveolar macrophages.
If the bacteria survive the initial encounter with the immune system the bacilli start replicating in the lungs, forming the pneumonic focus.
M. tuberculosis replicates slowly but continuously (doubling in number every 24 hours) and spreads via the lymphatic system to the hilar lymph nodes.
