Tuberculosis
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Direct answer
Caseating granulomas with epithelioid macrophages and Langhans giant cells are the histological signature of tuberculosis, a chronic granulomatous inflammation driven by type IV hypersensitivity to Mycobacterium tuberculosis. Primary infection in a non-immune host creates the subpleural Ghon focus plus draining hilar lymphadenopathy (together the Ghon complex), whereas post-primary disease reactivates in the lung apex, cavitates, and spreads by airways. India carries the world's largest tuberculosis burden, and the National Tuberculosis Elimination Programme has made upfront CBNAAT (GeneXpert) testing standard because it confirms diagnosis and detects rifampicin resistance within two hours.
What you must remember
- The tubercle: central amorphous caseous necrosis surrounded by epithelioid macrophages, Langhans giant cells (nuclei arranged peripherally in a horseshoe) and a rim of lymphocytes.
- Ghon focus = subpleural caseous lesion, usually lower or mid zone; Ghon complex = focus plus involved hilar nodes; Ranke complex = the healed, fibrocalcified version.
- Post-primary (reactivation) TB favours the apex (Simon focus) because of high oxygen tension; apical cavitation is the source of positive sputum and massive haemoptysis (bronchial artery erosion).
- Miliary TB is haematogenous dissemination producing innumerable 1-2 mm yellow-white foci in lungs, liver, spleen, bone marrow and meninges; chest X-ray may be deceptively normal early.
- Extrapulmonary TB: lymph node (scrofula) is the commonest site in India; Pott disease targets the lower thoracic spine; tuberculous meningitis coats the base of the brain with gelatinous exudate and favours cranial nerve III, VI and VII palsies.
- Smear microscopy (Ziehl-Neelsen) needs roughly 10,000 organisms per mL of sputum; Löwenstein-Jensen culture remains the conventional gold standard but takes 4-6 weeks; liquid culture (BACTEC) is faster.
- Mantoux reads induration at 48-72 hours: 10 mm is positive in most cases, 5 mm in immunosuppressed or HIV-infected persons; it detects infection, not active disease, and is often negative in miliary TB and advanced HIV because of anergy.
- Sarcoidosis is the great non-caseating mimic; absence of caseation with positive ACE levels and negative smear should stop you from labelling it tuberculosis.
From droplet nucleus to cavity — the pathology in sequence
Walk a primary infection through its natural history. An infected droplet nucleus of 1-5 microns reaches a terminal airway, typically in a lower-lobe subpleural location. For the first 1-3 weeks organisms multiply inside non-activated alveolar macrophages, and nothing is visible on imaging. Then CD4+ T-helper-1 cells arrive, secrete interferon-gamma, and convert macrophages into activated epithelioid cells that fuse into Langhans giants; the centre undergoes caseous necrosis. Systemic spread to hilar nodes and occult haematogenous seeding of the apex, bone marrow, kidneys and meninges occurs during this preimmune phase — which is exactly why these are the sites of later reactivation.
Most primaries heal as a fibrocalcific Ranke complex, and the Mantoux turns positive. Reactivation, classically with malnutrition, diabetes, steroids or HIV, occurs in the apical focus. Tissue immunity keeps organisms confined, so necrosis is more localised but more destructive: the caseum liquefies, erodes a bronchus, and is discharged to leave an apical cavity with foci of spread throughout both lungs via the airway. It is this liquefaction step, not the granuloma itself, that makes the patient infectious.
At the bench, request Ziehl-Neelsen smear, CBNAAT and culture on the same specimen. Biopsies of lymph node or synovium show the granuloma; acid-fast bacilli are scanty in extrapulmonary disease, so a compatible granuloma with negative smear still justifies antitubercular therapy while awaiting culture.
Where students slip
Two errors dominate viva answers. First, candidates call every granuloma tuberculosis: the correct sequence is to look for caseation, then stain for acid-fast bacilli, because fungal infections, leprosy (tuberculoid type) and sarcoidosis all produce granulomas without caseation. Second, the primary versus post-primary site is reversed under exam pressure — remember "primary below, reactivation at the apex," and that primary disease has prominent node involvement while reactivation disease has prominent cavitation. A classic image-based question shows a calcified peripheral lesion with calcified hilar nodes years after childhood infection; that is a Ranke complex, not active disease, and needs no treatment. Conversely, an elderly diabetic with an apical cavity and evening fevers is reactivation tuberculosis even if a remote childhood X-ray was reported normal.
Frequently asked questions
Which cell type is characteristic of the tubercle, and how are its nuclei arranged?
The Langhans giant cell, formed by fusion of epithelioid macrophages, with nuclei arranged peripherally in a horseshoe or wreath pattern; foreign-body giant cells have randomly scattered nuclei instead.
What distinguishes the Ghon complex from the Ranke complex?
The Ghon complex is the active subpleural caseous focus with ipsilateral hilar lymphadenopathy of primary infection; the Ranke complex is the same lesion healed by fibrosis and calcification.
Why does post-primary tuberculosis favour the lung apex?
High oxygen tension and impaired lymphatic drainage at the apex favour growth of the strictly aerobic organism, and apical seeding during the primary bacteraemia provides the focus that later reactivates (Simon focus).
Which investigation detects rifampicin resistance fastest?
CBNAAT (GeneXpert), a cartridge-based nucleic acid amplification test giving results in under two hours and simultaneously detecting M. tuberculosis and rifampicin resistance; it is recommended by the NTEP as the initial diagnostic test.
When is a 5 mm Mantoux reaction considered positive?
In HIV infection, immunosuppression, recent contacts and fibrotic lesions on chest X-ray; otherwise 10 mm is positive, and 15 mm was the older cut-off for low-risk persons.
What is the commonest extrapulmonary site of tuberculosis in India?
Tuberculous lymphadenitis, cervical nodes most often, showing caseating granulomas on fine-needle aspiration or biopsy.