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Post-primary Tuberculosis: Post-primary disease is that arising from infection in an individual previously sensitised with tubercle bacilli. The infection may be exogenous or endogenous (reactivation). About 10% of those previously infected with tubercle bacilli develop disease at some stage of their lives. The alteration to the host's immune status which occurs during primary infection modifies the response to reactivation of a primary lesion or to reinfection. Post-primary disease (occurring in the presence of acquired cellular immunity) is usually localised. Regional lymph node enlargement and dissemination of bacilli are uncommon. Extensive tissue destruction and necrosis with caseation can result from hypersensitivity to tuberculous proteins. Cavitating pulmonary lesions maintain the chain of infection by releasing a large number of bacilli in the sputum. Healing occurs by both fibrosis and calcification (calcification is extremely common in post-primary tuberculosis). Factors associated with an increased risk of reactivation include: diabetes, renal failure, old age, alcoholism, malnutrition, HIV infection leukaemia and lymphoma, silicosis and treatment with corticosteroids. Active tuberculosis may, however, develop in the absence of any of these conditions. The most commonly affected sites are the apical and subapical regions of the lungs, probably because of high intra-alveolar oxygen tension (tubercle bacilli are strict aerobes).
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Pathogenesis