III. Mediators of allergic reactions. Endogenous and exogenous stimulating or suppressor substances. Mediators of experimental hypersensitivity pneumonitis.

Using a physiologic model of hypersensitivity pneumonitis where depressions of arterial oxygen tension in unimmunized rabbits are monitored following aerosol challenge with Aspergillus terreus spores, attempts were made to assess the nature of the cellular and pharmacologic mediators of the impairment. Unlike normal animals no PaO2 depressions were obtained following aerosol challenge in either rabbits deficient in C6 or in rabbits made thrombocytopenic with antiplatelet serum. Such aerosols were also shown to produce platelet count depressions of up to 45% in normal rabbits. Finally, in vitro evidence of histamine release was obtained following incubation of Aspergillus extract, platelets and autologous serum from unimmunized rabbits. It was concluded that Aspergillus-induced pulmonary disease may be initiated by platelet release of mediators such as histamine stimulated by nonspecific complement activation.