Mast cells in human lungs

Mast cells were stained deeply in human lung tissue with acidic toluidine blue to obtain maximum numbers possible in paraffin sections. One hundred high‐power fields were counted per section, and mean and median values summarized as mast cells per mm 2 . Immersion‐fixed samples of fresh lung tissue (not bronchi) were taken as controls from seven patients after surgery, and showed mean values of 44.7 mast cells per mm 2 after formalin fixation, and 51.9 per mm 2 after Carnoy's fixative. Mast cell heterogeneity may explain these differences, but so could random variation between counts. In two patients with extrinsic allergic alveolitis (hypersensitivity pneumonitis), fresh lung tissue from open lung biopsies showed raised values of 90.8 and 101.9 mast cells per mm 2 , matching the high mast cell counts reported in bronchopulmonary lavage fluid in the condition. Control post‐mortem lung tissue from two patients dying of non‐pulmonary diseases showed mean values of 26.1 and 50.6 mast cells per mm 2 . Post‐mortem lung tissue from three patients dying of asthma showed very low mean values of 4.7, 5.7, and 5.9 mast cells per mm 2 . Low mast cell counts due to severe degranulation have been reported before in the bronchi in asthma deaths, but not, to our knowledge, in the lung parenchyma. This finding implies a wider area of mediator release, and helps to explain the severity of the acute attack, and the fatal outcome.