Background and Objectives: In immunocytochemical preparations of bronchoalveolar lavage (BAL) cells from patients with extrinsic allergic alveolitis (EAA), we observed the presence of alveolar macrophages with cockade-like structures in their cytoplasm ( cockade+ alveolar macrophages). These cockade+ alveolar macrophages may reflect a subpopulation of alveolar macrophages which may show a different predominance in various interstitial lung diseases. In this study we aimed to compare the frequency of cockade+ alveolar macrophages in patients with EAA ( n = 14) with the results obtained in patients with sarcoidosis ( n = 11), idiopathic interstitial pneumonia ( IIP; n = 10) and control subjects ( n = 8). We also investigated the expression of the transferrin receptor CD71 on cockade+ alveolar macrophages. Methods: In BAL fluid, the total number of cells and differential counts were determined, and immunocytologic examinations of macrophages and lymphocytes were done using monoclonal antibodies. The percentage of cockade+ alveolar macrophages was determined by counting 300 macrophages in the CD20 field of an immunocytochemical slide. Results: The percentage of cockade+ alveolar macrophages was significantly higher in the EAA group ( 36 +/- 9%) compared to patients with sarcoidosis ( 12 +/- 5%) or IIP ( 11 +/- 10%) and control subjects ( 3 +/- 1%; p < 0.001). The proportion of CD71+ alveolar macrophages was significantly lower in EAA than in the other groups ( p < 0.01), and the CD71 antigen was expressed on a significantly lower proportion of cockade+ alveolar macrophages compared to cockade - alveolar macrophages in EAA ( p < 0.001). Conclusion: We conclude that cockade+ alveolar macrophages could play a role in the pathogenesis and differential diagnosis EAA. Copyright (C) 2005 S. Karger AG, Basel.