Primary Infection

• The immune response (delayed hypersensitivity and cellular immunity) develops approximately 8-10 weeks after the primary infection.
• The pneumonic focus and related hilar lymphadenopathy form a primary complex.
• Bacilli may spread through the blood from the primary complex to other organs in the body of the infected person. The next phase is determined by the strength of the immune response of the infected person.
• Successful containment of TB is dependent on the cellular immune system, mediated primarily through T-helper cells (TH1 response). T cells and macrophages form a granuloma with a centre that contains necrotic material (caseous centre), M tuberculosis, and peripheral granulation tissue consisting primarily of macrophages and lymphocytes; the granuloma serves to prevent further growth and spread of M tuberculosis. These individuals are non-infectious and have latent TB infection; the majority of these patients will have a normal CXR and be tuberculin skin test (TST)-positive.