An investigation has been made of the process of local recovery occurring in the pulmonary lesions of dogs with experimental pneumococcus lobar pneumonia. Six animals showing simultaneous healing and spread of the pathological process in different parts of the lungs were sacrificed during the active stage of the disease for bacteriological and histological study. It was found that with few exceptions the clearing (resolving) lesions as revealed by x-ray during life were sterile on culture while the young, metastatic processes yielded an abundant growth of pneumococci. The resolving areas which represented the older lesions were characterized by the presence of a well developed macrophage reaction, whereas the early lesions consisted of alternating areas of edema and polymorphonuclear infiltration and contained usually many pneumococci both free and in the cells. The only pneumococci found in the areas of macrophage mobilization were intracellular forms, for the most part in the process of digestion. In several instances these striking differences in the cellular picture and the numbers and distribution of pneumococci were observed in different parts of a single lobe lesion. Tests on the blood serum failed to reveal any evidence of acquired antipneumococcal humoral immunity. The significance of these findings in relation to the mechanism of recovery is discussed.

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