Clinical features like fever; night sweats and weight loss and X-ray finding of a mass in apical lobe with histopathological findings of caseous necrosis are highly suggestive of tuberculosis. The underlying process is type IV hypersensitivity, which is characterized by the presence of modified macrophages, lymphocytes & giant cells. Schematic illustration of the events that give rise to the formation of granulomas in cell-mediated (type IV) hypersensitivity reactions. TuberculosisAntigenic property : Group specificity is due to polysaccharide while type specificity is due to a protein antigen. Antibodies are not useful for diagnosis and immunity.Pathogenicity:Macrophages are the primary cells infected by M. tuberculosis. M. tuberculosis replicated within the macrophages (phagosome) by blocking fusion of phagosome and Lysosome. It is due to the escape of killing by macrophages and induction of type IV hypersensitivity. Factors contributing to pathogenesis: Cord factor, Lipoarabinomannan, Complement system, M. tuberculosis heat shock proteins. Risk of acquiring infection is determined mainly by exogenous factors while the risk of developing disease depends largely on endogenous factors Most potent risk factor- HIV coinfection Clinical Features