Rabbits sensitized by injection were exposed to an aerosol of Micropolyspora faeni. Death occurred rapidly, and the lungs were immediately fixed and examined. The unprotected animals presented an allergic alveolitis with capillary lesions in which the changes in the endothelium were associated with intravascular coagulation. Heparin completely protected the endothelium but did not prevent the coagulopathy; an anti-platelet serum abolished this latter element of the reaction. The authors conclude that the alveolitis. which persisted in all the cases, was a Type 1 allergic reaction linked to the liberation of the mediators of immediate hypersensitivity.
The changes in the capillary endothelium are the consequence and not the cause of the intravascutar coagulation. This coagulation is independent of the Type 1 allergic phenomena and it appears to be responsible for the development of the fibrosing granulomata characteristic of “farmer's lung”.