Tubercle

Write a short note on the tubercle.
Answer:

Tubercle is formed by the tubercle bacilli.

Following is the sequence of events for the formation of tubercle bacilli:

As tubercle bacilli are ingested in the body they are lodged in pulmonary capillaries. Now neutrophils are evoked and they destroy the organisms.
After 12 hours progressive infiltration of macrophages takes place.
Macrophages start phagocytosing of tubercle bacilli and either kill bacteria or die. In the latter case, they further proliferate locally as well as there is increased recruitment of macrophages from blood monocytes.

Tubercle in tuberculosis

As a part of the body’s immune response, T and B cells are activated. Activated CD4+T cells develop the cell-mediated delayed-type hypersensitivity reaction, while B cells result in the formation of antibodies that play no role in the body’s defense against tubercle bacilli.
In 2-3 days, the macrophages undergo structural changes as a result of immune mechanisms—the cytoplasm becomes pale and eosinophilic and their nuclei become elongated and vesicular. This modified macrophage resembles epithelial cells and is called epithelioid cells.
The epithelioid cells aggregate into tight clusters or granulomas.
Some of the macrophages form multinucleated giant cells by fusion of adjacent cells.
Around the mass of epithelioid cells and giant cells is a zone of lymphocytes, plasma cells, and fibroblasts. The lesion at this stage is called a hard tubercle due to the absence of central necrosis.
Within 10-14 days, the center of the cellular mass begins to undergo caseation necrosis, characterized by a cheesy appearance and high lipid content. This stage is called as soft tubercle which is the hallmark of tuberculous lesions.
The soft tubercle which is a fully-developed granuloma with a caseous center does not favor the rapid proliferation of tubercle bacilli.

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