Organizing pneumonia (OP) is considered a non-specific repair process in lungs in response to various kinds of lung insults:

• cryptogenic organizing pneumonia (COP) when the cause is not known
• secondary organizing pneumonia when the cause is known
  • infection, inflammatory bowel disease, drugs (as in the above case), collagen vascular disease, organ transplantation and radiation direct to the chest

The histological hallmark of OP is intra-alveolar granulation tissue plugs (Masson bodies) which extend into bronchiolar lumina. The presence of these plugs is a prerequisite for restoration of tissue integrity.

Not to be confused with Bronchiolitis Obliterans Organizing Pneumonia (BOOP), which is a disease specifically of small airways.