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Nonspecific interstitial and cryptogenic organizing pneumonia

  • Lesion

Two interstitial patterns with a better outlook than IPF: NSIP is even inflammation or fibrosis; organizing pneumonia is fibroblast plugs in airspaces.

Updated

What it is

Nonspecific interstitial pneumonia (NSIP) is diffuse, uniform interstitial inflammation, fibrosis or both, often in younger adults and frequently linked to connective tissue disease. Organizing pneumonia is a healing pattern in which plugs of loose granulation tissue fill alveolar ducts and alveoli; when no cause is found it is called cryptogenic organizing pneumonia (COP). Both respond better to treatment than UIP.

Structure

  • Cellular NSIP: mild to moderate interstitial infiltrate of lymphocytes and a few plasma cells, spread evenly.
  • Fibrosing NSIP: diffuse or patchy interstitial fibrosis, all of the same age.
  • Organizing pneumonia: rounded, pale, myxoid plugs of fibroblasts and myofibroblasts in airspaces, with normal alveolar walls between them.
  • No hyaline membranes, granulomas or honeycombing in either.

Step by step

  1. An insult (infection, drug, autoimmune attack) injures the alveolar wall or airspace.
  2. In NSIP, inflammation spreads evenly through the septa at one time, so any fibrosis is uniform in age.
  3. In organizing pneumonia, fibrin exudate in the airspaces is invaded by fibroblasts instead of being cleared.
  4. The fibroblasts form plugs (Masson bodies) that obstruct ducts and bronchioles.
  5. Because the scaffold is preserved, the plugs can be resorbed with treatment and the lung returns to normal.

Markers

  • Masson bodies

Clinical notes

  • NSIP: subacute dyspnoea and cough in middle-aged adults, often women and non-smokers; search for connective tissue disease.
  • Cellular NSIP carries a much better prognosis than UIP; fibrosing NSIP is in between.
  • COP mimics pneumonia that does not respond to antibiotics.

Test yourself

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

What histologic feature separates fibrosing NSIP from UIP?

Fibrosis in NSIP is uniform and of the same age, without fibroblastic foci or honeycombing.

What are Masson bodies?

Polypoid plugs of loose organizing connective tissue (fibroblasts in a pale matrix) within alveolar ducts, alveoli and bronchioles, the mark of organizing pneumonia.

When is an organizing pneumonia called cryptogenic?

When no cause such as infection, drug, toxin, connective tissue disease or graft-versus-host disease is identified.

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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: Interstitial Lung Disease (StatPearls (NCBI Bookshelf), 2025); Interstitial Lung Diseases (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.