A study has been made of the occurrence of the macrophage reaction in the pulmonary lesions of dogs sacrificed during the course of experimentally induced pneumococcus lobar pneumonia or dying as a result of the infection. This characteristic transformation of the fixed tissue cells of the lung was found as a constant accompaniment of recovery. It was also present in varying degrees in the great majority of fatal instances provided the animal lived more than 40 hours. In general the longer the animal survived, the more pronounced the macrophage reaction observed in the lung lesions at autopsy. The numbers of pneumococci in the lesions diminished progressively with the evolution of the cellular change which terminated in resolution of the pneumonic exudate. Some dogs surviving for 4 days or more showed practically complete clearing of the pulmonary lesions but succumbed with an overwhelming bacteremia or empyema or both. On the other hand, several animals dying with a sterile blood, exhibited lesions characterized by little or no macrophage response and the presence of many pneumococci. These findings suggest that recovery from experimental lobar pneumonia in the dog depends on a dual mechanism consisting of a generalized process which prevents or controls invasion of the blood stream and a local one by which the lesion is finally freed from the invading microorganisms. The nature of these two processes is discussed.

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