The lung in immunocompromise and transplant
- Topic
When immunity is weak the lung is infected by organisms that rarely harm healthy people. The CD4 count predicts which ones in HIV.
What it is
Pulmonary infiltrates are among the commonest and most dangerous complications of immunosuppression from HIV, chemotherapy, transplantation or steroids. The type of defect points to the organism: neutropenia favours bacteria and moulds, T-cell defects favour Pneumocystis, viruses, mycobacteria and fungi. Lung transplant recipients add rejection, whose chronic form is bronchiolitis obliterans.
Structure
- Pneumocystis: alveoli filled with eosinophilic foamy material; organisms visible as small cup-shaped cysts with silver stain; mild interstitial plasma cells.
- CMV: enlarged cells with large intranuclear inclusions surrounded by a clear halo (owl's eye) and cytoplasmic inclusions.
- Invasive aspergillosis: necrotising nodules with target-like hemorrhage and hyphae invading vessel walls.
- Bronchiolitis obliterans: concentric fibrosis narrowing or closing bronchiolar lumens, patchy.
Step by step
- The immune defect removes the defence against a class of organism (neutrophils, T cells, antibodies).
- Opportunists already present (latent CMV, colonising Pneumocystis) or inhaled (Aspergillus spores) multiply freely.
- Without inflammation the picture is subtle; with immune recovery, inflammation can flare (reconstitution syndrome).
- In transplants, recipient T cells attack donor vessels and airways (acute rejection).
- Repeated injury of the small airways ends in fibrous obliteration (chronic rejection).
HIV: CD4 count and the lung infection to expect
| CD4 count | Typical infections | Note |
|---|---|---|
| Any count (above 200) | Bacterial pneumonia, tuberculosis | More frequent and severe than without HIV |
| Below about 200 | Pneumocystis jirovecii | Silver-stained cysts in foamy exudate |
| Below about 100 | Toxoplasma (brain), Cryptococcus | Cryptococcus can disseminate from the lung |
| Below about 50 | CMV, Mycobacterium avium complex, disseminated fungi | Advanced disease |
Clinical notes
- Pneumocystis: subacute dyspnoea, dry cough, fever, low oxygen with exertion; prophylaxis below the CD4 threshold.
- In HIV, bacterial pneumonias are more frequent and more often bacteraemic.
- Transplant recipients are most often infected between the first and twelfth month; chronic rejection affects half of lung recipients within a few years.
Test yourself
Think of the answer, then tap the question to see it.
Below what CD4 count does Pneumocystis pneumonia usually occur?
When the CD4 count is below about 200.
How do Aspergillus and Mucorales hyphae differ in tissue?
Aspergillus has thin septate hyphae with acute-angle branching; Mucorales has broad, ribbon-like, pauci-septate hyphae with right-angle branching.
What is the main morphologic correlate of chronic lung allograft rejection?
Bronchiolitis obliterans, fibrous obliteration of small airways.
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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: Aspergillosis (MedlinePlus, US National Library of Medicine, 2025); Pneumonia (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.