[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Lung](https://durgical.com/en/general-pathology/areas/lung/) › The lung in immunocompromise and transplant

# 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.

Updated Nov 12, 2026

## Key points

- HIV: bacterial pneumonia and TB at any CD4 count; Pneumocystis when the CD4 count falls below about 200; CMV, MAC and disseminated fungi below about 50.
- Pneumocystis: foamy, frothy pink alveolar exudate with cysts seen on silver stain; diffuse infiltrates, hypoxaemia.
- Aspergillus: septate hyphae branching at acute angles; invades vessels: hemorrhagic infarcts in neutropenic patients.
- Mucorales: broad, ribbon-like, non-septate hyphae with right-angle branching; diabetes with ketoacidosis, neutropenia.
- Not all infiltrates are infection: Kaposi sarcoma, lymphoma, lung cancer, drug reactions.
- Lung transplant: acute rejection = perivascular lymphocytes; chronic rejection = bronchiolitis obliterans (fibrous obliteration of small airways).

## 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)](https://durgical.com/en/conditions/community-acquired-pneumonia/) | Bacterial pneumonia, tuberculosis | More frequent and severe than without HIV |
| [Below about 200](https://durgical.com/en/conditions/pneumocystis-pneumonia/) | Pneumocystis jirovecii | Silver-stained cysts in foamy exudate |
| Below about 100 | Toxoplasma (brain), Cryptococcus | Cryptococcus can disseminate from the lung |
| [Below about 50](https://durgical.com/en/conditions/cytomegalovirus-infection/) | 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.

## Memory aid

Two hundred for Pneumocystis, fifty for CMV and MAC: the lower the count, the stranger the bug.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/general-pathology/the-lung-in-immunocompromise-and-transplant/)

[More Pathology questions →](https://durgical.com/en/practice/?f=pathology)

## Elsewhere in DurgiPedia

- [Transplant rejection: hyperacute, acute, chronic](https://durgical.com/en/immunology/transplant-rejection-hyperacute-acute-chronic/)

## Connections

## Related 4

- [Granulomatous pneumonia: tuberculosis and endemic fungi](https://durgical.com/en/general-pathology/granulomatous-pneumonia-tuberculosis-and-endemic-fungi/)
- [Viral pneumonia](https://durgical.com/en/general-pathology/viral-pneumonia/)
- [Community-acquired pneumonia: organisms](https://durgical.com/en/general-pathology/community-acquired-pneumonia-organisms/)
- [Oral infections and the mouth in systemic disease](https://durgical.com/en/general-pathology/oral-infections-and-the-mouth-in-systemic-disease/)

## Related conditions 5

- [Pneumocystis pneumonia (PCP)](https://durgical.com/en/conditions/pneumocystis-pneumonia/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Cytomegalovirus infection](https://durgical.com/en/conditions/cytomegalovirus-infection/)
- [Aspergillosis](https://durgical.com/en/conditions/aspergillosis/)
- [Kaposi sarcoma](https://durgical.com/en/conditions/kaposi-sarcoma/)

## Microorganisms 5

- [Pneumocystis jirovecii](https://durgical.com/en/microbiology/pneumocystis-jirovecii/)
- [Aspergillus fumigatus](https://durgical.com/en/microbiology/aspergillus-fumigatus/)
- [Cytomegalovirus](https://durgical.com/en/microbiology/cytomegalovirus/)
- [Cryptococcus neoformans](https://durgical.com/en/microbiology/cryptococcus-neoformans/)
- [Mucorales (Rhizopus, Mucor)](https://durgical.com/en/microbiology/mucorales/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

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](https://medlineplus.gov/aspergillosis.html) (MedlinePlus, US National Library of Medicine, 2025); [Pneumonia](https://medlineplus.gov/pneumonia.html) (MedlinePlus, US National Library of Medicine, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/general-pathology/the-lung-in-immunocompromise-and-transplant/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/the-lung-in-immunocompromise-and-transplant/