•The most common cause of TB is Mycobacterium tuberculosis.
•In patients with AIDS, a frequent causative agent is
•In underdeveloped countries, M. bovis, the causative agent of bovine tuberculosis causes gastro-intestinal TB and is acquired through the ingestion of contaminated milk.
•The bacterium enters the individual through inhalation of infectious
sputum; in many underdeveloped countries, ingestion of contaminated
milk is another source of infection.
•Irrespective of the portal of entry, the bacterium is ingested
by macrophages and induces an output of cytokines responsible
for the constitutional symptoms.
•Perhaps due to its hard, waxy coat, the bacterium resists
lysis by the usual intracellular killing mechanisms of the
•Multiplication within the macrophage follows, with subsequent lysis
of the host cell permitting spread of infection.
•Macrophages also travel to local lymph nodes, carrying the infection
to these structures.
•Development of an immune response aborts the spread of infection
through unknown mechanisms and results in the formation of the
•However, it appears that the mycobacteria are not killed within
•Later in life, if the level of immunity is lowered for any
reason, reactivation of infection occurs.
•At this stage, the cell mediated immunity or delayed type hypersensitivity (DTH) contributes to tissue destruction.
33% of the world^s population and is responsible
for 3 million deaths a year.
•In the first world, with the reduction in over-crowding and
improvements in sanitation and in the health care of
milk-cow herds, the incidence of the disease has fallen
through much of the 20th century.
•Recently, however, the reported incidence has climbed, mainly due to the frequency of the infection in patients with AIDS.
•Tuberculosis can result in symptoms in almost any
however, the most common manifestations relate to the respiratory
•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
•TB in lymph nodes causes lymphadenopathy.
•TB in the GI tract can manifest as obstruction.
|General Gross Description|
lesions may occur in almost any organ of the
•The most common sites are the lungs and lymph nodes.
•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.
|General Micro Description|
•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.
•CASEATION IS CHARACTERISTIC BUT NOT PATHOGNOMONIC OF TB.
•When this necrotic material communicates with the exterior, as in tuberculosis of the lung, it is coughed up and helps spread the infection.
• Cotran RS, Kumar V, Robbins SL: Robbins Pathologic Basis of Disease. 5th edition. Philadelphia, W.B. Saunders, 1994, pp. 324.