[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/) › Hypersensitivity pneumonitis

# Hypersensitivity pneumonitis

- Lesion

Repeated breathing of organic dust (mouldy hay, bird proteins) causes immune inflammation around the small airways, with loose granulomas and later fibrosis.

Updated Nov 12, 2026

## Key points

- Classic exposures: farmer's lung (thermophilic actinomycetes in mouldy hay), bird fancier's lung (bird proteins), humidifier and hot-tub lung.
- Triad: interstitial lymphocyte-rich infiltrate around bronchioles, poorly formed non-necrotising granulomas, and organizing pneumonia.
- Mechanism: immune complexes (type III, precipitating antibodies) plus Th1 granulomas (type IV).
- Acute form: fever, cough and dyspnoea four to eight hours after exposure; chronic form: insidious fibrosis that can look like UIP or fibrotic NSIP.
- Lavage: lymphocytosis with a low CD4/CD8 ratio (the opposite of sarcoidosis).

## What it is

Hypersensitivity pneumonitis is an immunologically mediated inflammation of the distal airways and alveoli caused by repeated inhalation of organic antigens. Unlike asthma, it is centred on the alveoli and bronchioles and is a mixed type III and type IV reaction. Recognising it early matters: removing the exposure can stop progression to chronic fibrosis.

## Structure

- Interstitial pneumonitis of lymphocytes, plasma cells and macrophages, accentuated around bronchioles.
- Loose, poorly formed granulomas and scattered giant cells, sometimes with cholesterol clefts, in two thirds of cases.
- Intraluminal plugs of organizing pneumonia in bronchioles.
- Chronic disease: upper-zone predominant interstitial fibrosis, sometimes honeycombing.

## Step by step

- A sensitised person inhales fine organic particles that reach the alveoli.
- Antigen meets circulating IgG antibody and forms immune complexes in the alveolar walls; complement is activated.
- Th1 and CD8 T cells respond to the antigen and drive loose granulomas.
- Each exposure renews inflammation; between exposures it may subside.
- Persistent exposure activates fibroblasts and leads to irreversible fibrosis.

## Hypersensitivity pneumonitis vs sarcoidosis

| Feature | Hypersensitivity pneumonitis | Sarcoidosis |
| --- | --- | --- |
| [Cause](https://durgical.com/en/conditions/hypersensitivity-pneumonitis/) | Inhaled organic antigen | Unknown antigen |
| [Granulomas](https://durgical.com/en/conditions/sarcoidosis/) | Poorly formed, loose, bronchiolocentric | Tight, well-formed, along lymphatics |
| Lavage CD4/CD8 | Low (CD8 predominant) | High (CD4 predominant) |
| Hilar nodes | Not prominent | Bilateral hilar adenopathy |
| Hypersensitivity type | III and IV | IV |

## Clinical notes

- Recurrent 'flu-like' episodes linked to work in the barn, keeping birds or a contaminated humidifier.
- Serum precipitins prove exposure but not disease; CT shows centrilobular nodules, ground glass and air trapping.
- Avoiding the antigen is the main treatment; chronic fibrotic forms have a worse outlook.

## Memory aid

Hay, birds and humidifiers: the three H-sources of hypersensitivity pneumonitis.

## Test yourself

Think of the answer, then tap the question to see it.

## Which antigen causes farmer's lung?

Spores of thermophilic actinomycetes in mouldy hay.

## What are the three histologic features of hypersensitivity pneumonitis?

Bronchiolocentric lymphocytic interstitial pneumonitis, poorly formed non-necrotising granulomas, and organizing pneumonia.

## How does the lavage lymphocyte profile differ from sarcoidosis?

Hypersensitivity pneumonitis shows lymphocytosis with a low CD4/CD8 ratio; sarcoidosis has a high ratio.

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## Elsewhere in DurgiPedia

- [Type III hypersensitivity (immune complexes)](https://durgical.com/en/immunology/type-iii-hypersensitivity/)
- [Type IV hypersensitivity (delayed, T cells)](https://durgical.com/en/immunology/type-iv-hypersensitivity/)

## Connections

## Related 4

- [Granulomatous inflammation: caseating vs non-caseating, the causes to know](https://durgical.com/en/general-pathology/granulomatous-inflammation-caseating-vs-non-caseating-the-causes-to-know/)
- [Sarcoidosis: the lung](https://durgical.com/en/general-pathology/sarcoidosis-the-lung/)
- [Nonspecific interstitial and cryptogenic organizing pneumonia](https://durgical.com/en/general-pathology/nonspecific-interstitial-and-cryptogenic-organizing-pneumonia/)
- [Asthma: pathology](https://durgical.com/en/general-pathology/asthma-pathology/)

## Related conditions 1

- [Hypersensitivity pneumonitis](https://durgical.com/en/conditions/hypersensitivity-pneumonitis/)

[← 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: [Interstitial Lung Disease](https://www.ncbi.nlm.nih.gov/books/NBK541084/) (StatPearls (NCBI Bookshelf), 2025); [Interstitial Lung Diseases](https://medlineplus.gov/interstitiallungdiseases.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/hypersensitivity-pneumonitis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/hypersensitivity-pneumonitis/