Tuberculosis is a chronic granulomatous disease caused by tubercle bacilli-Mycobacterial tuberculosis and mycobacterium bovis.It mostly affects the lungs but may affect any organ or tissue in the body except myocardium,skeletal muscle,pancrease and thyroid.

Virulence of mycobacteriam depends on the cell wall mycosides.Cord factor is a derivative of mycosides.Mycosides interfere phagolysosomal fusion and thus allow the intracellular survival in macrophages. Delayed hypersensitivity develops in 2 to4 weeks after initial infection.

Two types of host response can occure in tuberculosis: 1)Primary tuberculosis. 2)Secondary tuberculosis. Primary pulmonary tuberculosis: It occurs after initial infection with mycobacterial tuberculosis or mycobacterial bovis.Ghon complex is the lesion of primary tuberculosis and consists of : 1)Ghon focus:Primary tissue lesion usually in the lung.

In the lung a subpleural lesion,often just above or just below the interlober fissure between the upper and lower lobes.1 to 5 cm gray- white. Fates: 1) Healing with ulcer and calcification. 2) Progressive primary tuberculosis.:Prrogressive spread very rarely cause tuberculous pneumonia,cavitation and miliary tuberculosis in infants,children and immunodeficiency adults.

Secondary tuberculosis: Secondary tuberculosis refers to all forms of active infection that occur in a sensitized individual to the mycobacterium. (1)Reactivation of dormant bacilli from primary lesion . (2)Reinfection of tubercle bacilli or (3)Directly from primary lesion. Secondary primary tuberculosis lesion is usually located in the apex of the lung.

Initial lesion in the lung is a small focus of consolidation less than 3 cm in diameter. Progressive pulmonary tuberculosis: Progression of lesion may lead to cavitary tuberculosis,tuberculous pneumonia,endobronchial and endotracheal tuberculosis,pulmonary miliary tuberculosis due to spread of tubercle bacilli through lymphatics to vena cava to lung,systemic tuberculosis due to spread of tubercle bacilli through pulmonary veins and isolated organ tuberculosis(e.g:meninges,cervical lymph node,kidneys,adrenals,fellopian tubes,epididymis,bones).