In vivo, IgE production is related to bronchial hyperresponsiveness and, in vitro, passive sensitization of human airways with asthmatic serum containing a high concentration of IgE enhances the contractile response to a variety of agonists. However, cell types implicated in this IgE sensitization are not fully determined. The aim of this study was to determine IgE-bearing cells during passive sensitization with special reference to mast cells. Peripheral bronchi were dissected out from 10 lung specimens obtained at thoracotomy and processed into glycolmethacrylate resin. Sections, each 2 microm thick, were passively sensitized by incubation for 2 h at 37 degrees C in either buffer supplemented with monoclonal IgE or asthmatic serum with a high concentration of IgE (> or = 1,000 IU/ml). Immunohistochemistry was performed using monoclonal antibodies directed against the epsilon chain, and markers of the various IgE-bearing cells (e.g., AA1, antichymase). The number of IgE-bearing cells was significantly higher in passively sensitized specimens as compared with nonsensitized specimens (6.63 +/- 1.71 versus 4.29 +/- 1.35/mm2; p = 0.013, n = 10). Mast cells represented 65% of IgE-bearing cells, 41.6 and 23.4% for TC and T subtypes, respectively. These results indicate that mast cell is the main cell type involved in IgE-induced passive sensitization. The involvement of mast cell-derived tryptase in the mechanisms of IgE-related hyperresponsiveness should be further examined.