Michael P. D'Alessandro, M.D.
Peer Review Status: Internally Peer Reviewed
The initial infection with the bacterium Mycobacterium Tuberculosis is acquired via an airborne route. If the initial infection is not walled off, it may spread via bronchogenic spread with multiple pulmonary foci, lymphatic spread to more distant lymph nodes and hematogenous (miliary) spread to kidneys, lungs, liver, and meninges.
The classic finding is caseating necrosis.
The initial focus has localized air space consolidation and then spreads from the periphery via lymphatic channels to regional lymph nodes where the lymph nodes enlarge after several weeks. If the infection involutes the lesion can calcify with the Ghon focus being the calcified peripheral lung focus and the Ranke complex being the calcified peripheral lung focus and the calcified central lymph nodes. If resistance fails to develop and there is no involution the initial peripheral focus can extend to occupy an entire segment or lobe. Often there may be an associated pleural effusion. Primary TB has its peripheral foci scattered throughout the lung fields. Secondary, or reinfection, TB has 90% of its lesions in apices. The lungs may take 4 years to clear after therapy. The classic picture of primary TB is air space disease associated with hilar/mediastinal adenopathy and pleural effusion.
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