Primary Tuberculosis

When the host encounters with M. tuberculosis for the first time, the following events occur:

• 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 and intestines.