Fig. 1: Nonfibrotic hypersensitivity pneumonitis (HP).

A High resolution CT scan showing multiple centrilobular ground glass nodules and hyperlucent area representing air trapping (arrow). B High resolution CT scan showing the triple density pattern: hyperlucent areas representing air-trapping, normal lung, and ground glass opacities. There is also mild reticulation. C Low power view showing mild bronchiolocentric chronic interstitial inflammation and interstitial granulomas. This is the usual pattern of nonfibrotic HP. D Marked chronic inflammation of bronchiolar wall with granulomas representing so-called “bronchiolitis” in nonfibrotic HP. The process focally extends into the interstitium. Narrowing of the bronchiolar lumen by inflammation in the wall is the cause of air-trapping on CT imaging. E Higher power view of Fig. 3 showing details of the granuloma; one is interstitial but a second granuloma is in an airspace. Although granulomas in HP are often said to be poorly defined, in fact in many cases they are quite well defined, as here, but always lack the concentric fibrosis seen in many sarcoid granulomas.