The size of the lesion can vary from a few millimeters to the entire lobe of the lung or the entire node. Early lesions are characterized by small, white, soft nodules a few mm. in diameter. Larger lesions, especially in the lung are characterized by extensive necrosis and cavitation. The walls of the cavities contain ragged, white material that houses millions of mycobacteria and is intensely infectious. Late, healed lesions can be walled off by fibrosis and appear white and hard.
Dystrophic calcification can also take place, giving rise to a white chalky appearance.
Irrespective of the organ affected, tuberculosis is characterized by granulomas, with epitheloid cells and Langhans giant cells. Very early lesions may be non-caseating, but typically tubercular granulomas undergo central caseation necrosis. When this necrotic material communicates with the exterior, as in tuberculosis of the lung, it is coughed up and helps spread the infection. Mycobacterium tuberculosis causes secretion of cytokines by macrophages, resulting in the typical constitutional reactions of fever and weight loss. Spread of disease to the entire lobe of the lung, especially with the development of communcation with the exterior through the bronchial tree results in coughing, often with blood tinged sputum.