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# Idiopathic pulmonary fibrosis (UIP pattern)

- Lesion

IPF is progressive scarring of the lung in older adults. Repeated injury to the alveolar lining heals badly, and the scar spreads from the bases.

Updated Nov 12, 2026

## Key points

- UIP pattern: patchy (spatial heterogeneity) fibrosis with lesions of different ages (temporal heterogeneity).
- Fibroblastic foci: the earliest active lesions, bluish myxoid fibroblast nests under the epithelium.
- Honeycombing: cystic spaces lined by bronchiolar epithelium within dense scar; subpleural and lower lobes first.
- Risk: smoking, older age, telomere genes (TERT, TERC), MUC5B promoter variant, surfactant protein mutations.
- Clinical: dry cough, exertional dyspnoea, fine 'velcro' crackles, clubbing; restrictive pattern with low DLCO.

## What it is

Idiopathic pulmonary fibrosis is a progressive fibrosing disease of unknown cause, seen almost always after 50, with respiratory failure as the end point. Its histologic pattern is usual interstitial pneumonia (UIP). The leading idea is repeated injury of alveolar epithelium in a person whose cells age or repair badly. UIP can also occur in connective tissue disease, chronic hypersensitivity pneumonitis and asbestosis, which must be excluded.

## Structure

- Gross: cobblestoned pleura from retracted septal scars; firm, rubbery white fibrosis in lower lobes, subpleural areas and interlobular septa.
- Microscopy: patchy interstitial fibrosis with normal lung next to effaced lung; collagen of different ages side by side.
- Fibroblastic foci at the edge of dense scar; honeycomb cysts lined by bronchiolar or hyperplastic type II cells, often full of mucus.
- Smooth muscle metaplasia, mild inflammation, and pulmonary arterial changes of secondary hypertension.

## Step by step

- Smoke, pollution, microaspiration, metal or wood dust repeatedly injure alveolar epithelial cells.
- Cells with short telomeres or misfolded surfactant proteins become senescent or stressed and fail to regrow the lining.
- Injured epithelium and alternatively activated macrophages release TGF-β and other fibrogenic factors.
- Fibroblasts proliferate (fibroblastic foci), become myofibroblasts and lay down collagen.
- New injuries keep occurring, so new foci form beside old scar: temporal heterogeneity.
- Scar remodels alveoli into honeycomb cysts; gas exchange fails, pulmonary hypertension follows.

## UIP vs NSIP vs organizing pneumonia

| Feature | UIP (IPF) | NSIP | Organizing pneumonia |
| --- | --- | --- | --- |
| Distribution | Patchy, subpleural, basal | Diffuse, uniform | Patchy airspace |
| Age of lesions | Mixed (temporal heterogeneity) | Same age (uniform) | Same age |
| Fibroblastic foci | Present | Absent | Polyps of fibroblasts in airspaces (Masson bodies) |
| Honeycombing | Yes | No | No |
| Outlook | Poor | Better | Usually good with steroids |

## Markers

- Fibroblastic foci
- Honeycomb change

## Clinical notes

- Insidious onset in older adults; median survival without transplant is only a few years.
- High-resolution CT showing subpleural, basal reticulation with honeycombing can be diagnostic without biopsy.
- Acute exacerbations show diffuse alveolar damage on top of UIP; antifibrotic drugs slow decline.

## Memory aid

UIP = Uneven In Phase: patchy and of many ages, with foci where it is still happening.

## Test yourself

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

## What does temporal heterogeneity mean in UIP?

Fibrotic lesions of different ages sit side by side: active fibroblastic foci next to old dense collagen scar.

## Which genetic factors increase the risk of IPF?

Mutations in telomere maintenance genes (TERT, TERC and others), a MUC5B promoter variant and, rarely, surfactant protein mutations.

## Which three other conditions can produce a UIP pattern?

Connective tissue diseases, chronic hypersensitivity pneumonitis and asbestosis.

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## Connections

## Related 6

- [Fibrosis in organs: TGF-β and the myofibroblast](https://durgical.com/en/general-pathology/fibrosis-in-organs-tgf-and-the-myofibroblast/)
- [Cellular aging: telomeres, DNA damage, senescence and proteostasis](https://durgical.com/en/general-pathology/cellular-aging-telomeres-dna-damage-senescence-and-proteostasis/)
- [Nonspecific interstitial and cryptogenic organizing pneumonia](https://durgical.com/en/general-pathology/nonspecific-interstitial-and-cryptogenic-organizing-pneumonia/)
- [Asbestos-related disease](https://durgical.com/en/general-pathology/asbestos-related-disease/)
- [The lung in autoimmune disease](https://durgical.com/en/general-pathology/the-lung-in-autoimmune-disease/)
- [Obstructive vs restrictive lung disease: the comparison table](https://durgical.com/en/general-pathology/obstructive-vs-restrictive-lung-disease-the-comparison-table/)

## Related conditions 1

- [Interstitial lung disease](https://durgical.com/en/conditions/interstitial-lung-disease/)

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

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