When the host encounters with M. tuberculosis for the first time, the following
- Settlement in alveoli.
- Local inflammatory response -Encounter with neutrophils and resident macrophages.
- Transport of engulfed bacilli (resident macrophages or neutrophils can not kill
tubercle bacilli) to regional lymph nodes. They continue to multiply intracellularly.
- Killing of the phagocytosed tubercle bacilli occurs only when macrophages are
activated. (Recall Immunology lectures - how cellular responses to certain bacteria and
lymphokines activate macrophages). Simultaneously, the development of cell-mediated
immunity, and delayed type hypersensitivity to tuberculin (antigenic protein(s) produced
by M. tuberculosis.
- If the host defense system fails to contain the spread of the bacilli at this
point, the following events ensue. It is estimated that approximately 10% of individuals
with normal immunity well develop active TB within their lifetime, 5% within the first 2
years of infection.
- Caseation of lymph nodes.
- Spread of organisms within the host: Tubercle bacilli spread in the host by
direct extension, through the lymphatic channels and bloodstream, and via the bronchi and
gastrointestinal tract. These events result in tuberculous meningitis, miliary
tuberculosis or both.
- If a caseating lesion discharges its contents into a bronchus, they are aspirated
and distributed to other parts of the lungs or are swallowed and passed into the stomach