The risk of developing TB disease after being infected depends on the individuals state of health, age, strength of immune system. Infectious droplet nuclei are released into air when patients with TB of lungs cough, sneeze or spit. Tubercle bacilli usually enter the lung via airways and reach the alveolar spaces in the lungs. Neutrophils present in the blood are responsible for phagocytosis initially and macrophages are recruited later. The TB organisms are phagocytosed and majority of them are killed. This is the stage I of disease. However a small proportion of bacilli will survive and replicate within macrophages and cause cell death (Stage II). Acid fast bacilli are seen in this stage. TB bacilli are transported via the lymphatics to the regional lymph node in which granuloma develops (Stage III). The initial focus of caseous bronchopneumonia together with lymphadenopathy is called Ghon complex. From the lymphatics, bacilli enter the veins and spread to other parts of the body including the lung, the brain, kidney and the bones.