The cells in the early interstitial infiltrate consist of lymphocytes, macrophages and scarce plasma cells. Certain areas within the interstitial pneumonitis show prominent vessel involvement. Arterioles are the most involved vessels, showing subintimal proliferation with infiltration of macrophages, lymphocytes, plasma cells and occasional eosinophils into the vessel wall. The necrotic areas can be scattered or may be localized to one area. Histologically these lesions contain caseous necrosis and will eventually be absorbed or become calcified.
Organisms are rare and tend to be present in the necrotic areas.
The chronic cavitary lesions have a thick wall of 2-3 mm with a necrotic inner lining. This is surrounded by highly vascular granulation tissue around which there is a fibrous capsule. These are associated with poor prognosis. The initial interstitial infiltrate usually heals over the course of several weeks and has excellent prognosis. Enlarging cavities and fibrosis lead to progressive lung destruction and respiratory failure over months or years.