Definition
Pulmonary TB is an infectious disease caused by Mycobacterium tuberculosis. It primarily affects the lungs, but may also involve the bones, joints, kidneys, peritoneum, and genital system.
M. tuberculosis: Key Features
- Obligate aerobes → prefer oxygen-rich areas like the lungs.
- Intracellular pathogens → survive inside macrophages; explains the strong T-cell immune response and lymphocyte increase in patients.
- Acid-fast bacilli → stainable with Ziehl-Neelsen stain for diagnosis.
TB Inflammatory Process: Special Characteristics
- Tubercles → small granulomas formed of epithelioid cells, Langhans giant cells, and lymphocytes.
- Caseous necrosis → cheese-like central necrosis, may liquefy to form cavitation.
- Calcification during healing → common feature as lesions resolve.
Immunity in TB
- T-cell mediated immunity → crucial in defense.
- Natural immunity = asymptomatic carriers.
- Artificial immunity = BCG vaccination (contains attenuated tuberculous bacteria, which act only as an Ag w/out causing a disease.)
- Transmission: via inhalation of droplets containing M. tuberculosis.
Primary Tuberculosis
- Occurs shortly after initial exposure to TB bacilli.
- Affects middle & lower lobes (site of maximum air distribution).
- Develops Ghon focus (acute exudative lesion) → may spread to regional lymph nodes.
- Hilar or paratracheal lymphadenopathy appears.
- Caseation & calcification of lymph nodes commonly follow.
- The nonspecific inflammatory response evoked on 1st exposure to tubercle bacilli becomes granulomatous, evoking the formation of tubercles.
- Ghon complex = combo of Ghon focus (lung lesion) + nearby lymph node swelling ± pleural thickening.
- TB bacteria can silently spread through blood (hematogenous dissemination) to other body parts.
- After body develops immunity, TB usually becomes latent (silent) in most people (90%) with no symptoms.
- In young kids or weak immune systems, TB may progress—either at the lung or other body sites reached early.