Granulomatous pneumonia: tuberculosis and endemic fungi
- Lesion
Tuberculosis and fungi such as Histoplasma and Paracoccidioides cause slow pneumonias made of granulomas, often with caseous necrosis, cavities and scars.
What it is
Chronic pneumonia in an immunocompetent person is usually a localised granulomatous inflammation caused by Mycobacterium tuberculosis or dimorphic fungi. The lung lesion follows the same logic for all: a Th1 response builds granulomas that contain the organism, sometimes with caseous necrosis, cavitation and calcified scars. Tuberculosis, histoplasmosis and paracoccidioidomycosis are all relevant in Colombia.
Structure
- Caseating granuloma: central cheese-like, structureless eosinophilic necrosis, epithelioid histiocytes, Langhans giant cells and a lymphocyte cuff.
- Cavity: caseous material discharged into a bronchus leaves a hole with a fibrous wall, full of bacilli.
- Miliary pattern: countless uniform small yellow-white nodules.
- Ziehl–Neelsen stain shows acid-fast bacilli; Grocott methenamine silver and PAS show fungi.
Step by step
- Inhaled bacilli or fungal spores are engulfed by alveolar macrophages but survive inside them.
- Organisms multiply and spread to hilar nodes before immunity develops.
- After a few weeks Th1 cells release IFN-γ; activated macrophages form granulomas that contain the organism.
- Caseous necrosis forms in the centre; most primary foci heal and calcify, leaving latent organisms.
- If immunity falls (HIV, diabetes, malnutrition, steroids), organisms reactivate in the apex and cavities form.
- Erosion into a bronchus spreads disease and makes the patient contagious; erosion into vessels causes hemoptysis or miliary spread.
Granulomatous lung infections: what the pathologist looks for
| Organism | Form in tissue | Stain | Clue |
|---|---|---|---|
| Mycobacterium tuberculosis | Thin rods inside caseous granulomas | Ziehl–Neelsen (acid-fast) | Apical cavities, Ghon complex |
| Histoplasma capsulatum | Tiny yeasts packed in macrophages | Silver, PAS | Caves, bat or bird droppings; calcified nodules |
| Paracoccidioides | Large yeast with many buds (pilot's wheel) | Silver | Rural workers in Latin America; oral ulcers |
| Coccidioides | Spherules full of endospores | Silver, PAS | Arid regions of the Americas |
| Blastomyces | Thick-walled yeast with broad-based bud | Silver, PAS | Skin and bone spread |
Markers
- Ziehl–Neelsen
- Grocott methenamine silver
- PAS
- IFN-γ release assay
Clinical notes
- Reactivation TB: chronic cough, hemoptysis, fever, night sweats and weight loss, with upper-lobe cavities.
- Histoplasmosis is usually silent; heavy exposure or HIV causes acute or disseminated disease.
- Paracoccidioidomycosis typically presents with lung disease and painful mulberry-like oral ulcers in rural workers.
- Colombia reports thousands of TB cases each year; HIV co-infection and drug resistance are concerns.
Test yourself
Think of the answer, then tap the question to see it.
What is the Ghon complex?
The primary TB lesion: a subpleural Ghon focus in the mid lung plus the involved hilar lymph nodes; when calcified it is the Ranke complex.
Why does reactivation tuberculosis favour the lung apices?
The apices have high oxygen tension, which favours growth of the aerobic bacilli.
What is the typical tissue form of Paracoccidioides?
A large yeast with multiple narrow-based buds around it, the pilot's (ship's) wheel.
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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Tuberculosis (MedlinePlus, US National Library of Medicine, 2025); Histoplasmosis (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.