Other Histologic Features of Hypersensitivity Pneumonia

Figure 1. Severe Alveolitis: In some cases, the peribronchiolar alveolitis can be severe with many alveolar macrophages, as well as widened alveolar walls and type II cell hyperplasia. This pattern probably corresponds to the nodules seen on the HRCT shown at the beginning of Unknown 12.


Note that the architecture is preserved. An interlobular septum lies at the right. The center of the acinus is marked by the two arteries. This inflammation can resolve or organize and lead to fibrosis.

Find two arteries marking a centriacinar location.

Figure 2. Organization

Alveolar exudate may organize rather than resolve. Here, granulation tissue has replaced alveolar exudate as shown above. The respiratory bronchiole to the left is not involved.

Airway organization and scarring, in contrast with alveolar organization, can lead to airflow obstruction that may be found in these patients.

Figure 3. Endogenous Lipoid Pneumonia: Foamy macrophages, shown here in alveoli in the bottom half of the image, may accumulate in alveolar spaces when there is impairment of alveolar clearance. The lipoid material is in small droplets rather than the large droplets seen in exogenous lipoid pneumonia (see Unknown 6).



Find an alveolus with foamy macrophages.


Note also the widened alveolar walls with scattered lymphoid cells and type II cell hyperplasia. A lymphoid aggregate lies next to the artery.

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Centriacinar artery





























Alveolus filled with foamy macrophages