Interstitial pneumonia is synonymous with atypical pneumonia. In contrast to typical pneumonia, interstitial pneumonia results in an inflammation of the alveolar walls while the alveolar air spaces remain relatively clear.
In this specimen note that the walls of the alveoli are thickened and filled with a cellular infiltrate while the alveolar air spaces are relatively uninvolved. Note also that the walls of the alveoli are lined with hyaline membranes. These membranes are a typical feature of interstitial pneumonia and are due to diffuse alveolar damage (DAD), which may produce the clinical picture of Acute Respiratory Distress Syndrome (ARDS).