Pathogenesis and Pathological Changes Tubercle bacilli get entry in to the human body through various
routes; but of these respiratory, gastro-intestinal and cutaneous routes are common. When the child is exposed to a diseased
individual, via droplet infection the organism enters the respiratory tract. Because of adequate ventilation lower two-
thirds of the lungs are the common sites for lodgement. The organisms are ingested by macrophages when they either die
or multiply, and ultimately are carried to the regional lymph nodes and then to other organs of the body through blood
stream before specific immunity develops. During this period as they produce no toxin and no general reaction, so no
symptom is present. Afterwards, the characteristic tissue reaction develops anywhere according to the site of lodgement
of the organism. This consists of central necrotic area undergoing caseation, epitheloid cells, Langhans type of giant
cells and a lymphocytic barrier at the periphery. This is the characteristic lesion of tuberculosis. When caseation develops,
this is associated with liberation of various cytotoxic materials from T lymphocytes giving rise to marked reduction in the
nnnulation of the organism. At the same time these toxic

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