Interpretation of pleural biopsy specimens and aspirates with the immunoperoxidase technique.

In pleural biopsy specimens and histological sections from the fibrin clots of pleural fluid aspirates it may be difficult to distinguish reactive mesothelial cells from malignant mesothelial cells and metastatic carcinoma. Reactive pleurisy with effusion is usually associated with loss of cohesion and exfoliation of mesothelial cells, which is consistent with the hypothesis that they act as facultative histiocytes. A series of biopsy specimens and sections of clots from benign and malignant pleural effusions have been stained by the immunoperoxidase technique for the histiocytic markers alpha 1-antitrypsin, alpha 1-antichymotrypsin, and lysozyme (muramidase). Eight cases of mesothelioma were included. Mesothelial cells when seen as a monolayer lining the pleural surface were negative. Reactive mesothelial cells, usually seen as exfoliated cells, were consistently strongly positive for alpha 1-antichymotrypsin and more variably for alpha 1-antitrypsin and lysozyme. Malignant cells, whether from carcinoma or from mesothelioma, were usually but not always negative. Consequently immunohistochemical staining for alpha 1-antichymotrypsin is often helpful in distinguishing reactive mesothelial cells from malignant cells.