The basic lesions of pulmonary tuberculosis include exudative lesions, proliferative lesions, and caseous necrosis. Exudative lesions are characterized by congestive edema of the tissue, infiltration of neutrophils, lymphocytes, and monocytes, fibrin exudation, and a small number of epithelioid cells, multinucleated giant cells, and antacid staining for Mycobacterium tuberculosis. The proliferative lesion is mainly manifested as a tuberculous nodule with a central multinucleated giant cell derived from macrophages, a giant cell with multiple nuclei arranged in a ring or trapezoid shape at one or both ends of the cell, surrounded by epithelioid cells transformed from macrophages in a laminar arrangement, with scattered lymphocytes and plasma cells at their outermost side. Caseous necrosis, the stage of further development and deterioration of exudative lesions, often appears as a yellow or yellow-white cheese-like solid, or as a semi-solid state.