Bronchoalveolar mast cells in summer-type hypersensitivity pneumonitis: increase in numbers and ultrastructural evidence of degranulation.

To investigate the role of mast cells in the development of summer-type hypersensitivity pneumonitis (SHP), which is the most common type of hypersensitivity pneumonitis in Japan, bronchoalveolar lavage fluid (BALF) cells from 13 patients with this disease was studied by light and electron microscopy. The proportion and number of BALF mast cells recovered from SHP patients were significantly greater than those from patients with sarcoidosis and from control subjects. The BALF mast cells from the SHP patients showed various ultrastructural changes such as the transformation of granules, cytoplasmic vacuolation, and an increase of lipid droplets. These findings appeared to reflect activation and degranulation of bronchoalveolar mast cells, suggesting that mast cells play a role as inflammatory cells in the development of SHP.