What is Tuberculosis?
Tuberculosis (TB) is a chronic infectious disease caused by the bacterium
Mycobacterium tuberculosis. It primarily affects the lungs but can also involve other organs. In the context of
Histology, TB is characterized by specific cellular changes and tissue reactions.
Histological Features of Tuberculosis
The hallmark of TB in histological examination is the formation of
granulomas. Granulomas are organized collections of immune cells that form as a response to chronic infection. These granulomas typically consist of the following components:
- Central area of
caseous necrosis: This is a cheese-like material resulting from the death of infected tissues.
- Surrounding layer of
epithelioid cells: These are activated macrophages that appear elongated.
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Langhans giant cells: These are multinucleated cells formed by the fusion of macrophages and are often seen in granulomas.
- Peripheral ring of
lymphocytes: These immune cells surround the granuloma.
How is Tuberculosis Diagnosed Histologically?
Diagnosis of TB through histology involves examining tissue samples under a microscope. Common methods include:
-
Ziehl-Neelsen stain: This special staining technique is used to identify acid-fast bacilli, which are indicative of Mycobacterium tuberculosis.
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Hematoxylin and Eosin (H&E) stain: This general staining method highlights the granuloma structure and caseous necrosis.
- Immunohistochemistry: Antibodies specific to mycobacterial antigens can be used to detect the presence of the bacteria.
Pathogenesis and Cellular Response
After inhalation, the bacteria reach the alveoli in the lungs, where they are phagocytosed by macrophages. However, Mycobacterium tuberculosis can survive and replicate within these
macrophages. The immune response involves the recruitment of more macrophages,
T-cells, and other immune cells to form granulomas, attempting to contain the infection.
Progression and Complications
In some cases, the immune system successfully contains the bacteria within granulomas, leading to a latent TB infection. However, if the immune response weakens, the granulomas may break down, releasing the bacteria and causing active TB. This can lead to widespread tissue damage and systemic dissemination, known as miliary TB.Histological Differences Between Primary and Secondary TB
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Primary TB: This initial infection is characterized by the formation of a
Ghon complex, which includes a granuloma in the lung parenchyma and a corresponding granuloma in the regional lymph nodes.
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Secondary TB: Also known as reactivation TB, it often occurs in the apical regions of the lungs. Histologically, it shows more extensive caseous necrosis and cavitation.
Histological Examination in Extrapulmonary TB
TB can also affect organs other than the lungs, such as lymph nodes, kidneys, bones, and the central nervous system. In these cases, histological examination reveals granulomas similar to those in pulmonary TB, often with caseous necrosis.Treatment and Histological Changes Post-Treatment
Antitubercular therapy aims to eradicate the bacteria and resolve the infection. Histologically, successful treatment results in the reduction of granulomas and inflammation. However, residual fibrosis and calcification may persist, indicating previous infection sites.Conclusion
Histology plays a crucial role in the diagnosis and understanding of tuberculosis. The presence of granulomas, caseous necrosis, and specific cellular responses are key histological features that help pathologists identify and confirm TB. Advanced staining techniques and immunohistochemistry further aid in the detection and study of Mycobacterium tuberculosis, contributing to effective diagnosis and treatment.
