primary tuberculosis

Primary TB occurs when an immunologically naive individual inhales droplet nuclei containing M. tuberculosis. Alveolar macrophages phagocytose the bacilli, but the organism can survive and replicate intracellularly due to inhibition of phagolysosome fusion. In most immunocompetent hosts (~90-95%), T-cell mediated immunity (Th1 response, IFN-gamma activation of macrophages) develops within 2-8 weeks, forming granulomas (caseating) that wall off the infection—this is the Ghon focus, typically located in the mid to lower lung zones (well-aerated areas) subpleurally. Combined with caseating hilar/mediastinal lymph node involvement, this is called the Ghon complex; when calcified, it is termed the Ranke complex. In most cases, primary TB is asymptomatic or presents with mild flu-like symptoms and resolves, with the bacteria remaining dormant within granulomas (latent TB infection, LTBI). A positive PPD/TST or IGRA indicates prior exposure and immune sensitization. In a minority (~5-10%), especially immunocompromised hosts (HIV, malnutrition, young children, elderly), primary TB can progress directly to active disease (progressive primary TB), potentially with miliary spread (hematogenous dissemination) or primary TB pneumonia. Unlike secondary (reactivation) TB, which occurs in previously sensitized individuals and favors apical lung segments (due to higher oxygen tension favoring aerobic mycobacterial growth) and causes cavitation, primary TB usually lacks cavitation because the immune response, while forming granulomas, is not yet vigorous enough to cause the tissue destruction seen in reactivation. Clinical significance: understanding primary vs secondary TB helps interpret radiographic findings (Ghon complex vs apical cavitary lesions), guides the use of TST/IGRA for screening, and is essential for recognizing which patients are at risk for reactivation (via loss of immune control, e.g., TNF-alpha inhibitors, HIV) later in life.