In view of the finding that one or more attacks of pneumococcus lobar pneumonia experimentally induced in dogs failed to protect the animals against subsequent infection, an attempt was made to determine whether or not the dog's antipneumococcal immunity could be enhanced to the degree of complete resistance to the experimental disease. To this end dogs were passively immunized by the intravenous injection of large quantities of both unconcentrated antipneumococcus horse serum and concentrated antibody solution and actively immunized by vaccination with killed and living cultures of pneumococci. None of these procedures were found to result in constant protection against the pulmonary infection. The disease, however, was of brief duration, the lesions of limited extent and usually sterile within 24 hours. A combination of active and passive immunization produced no better results. It was only when immune bodies and leucocytes were implanted with the infecting dose that prevention of infection was secured with any degree of constancy. Even under these conditions the lesion sometimes involved a considerable portion of a lobe. The factors involved in the inception of experimental lobar pneumonia are discussed and the bearing of this study on the prophylactic immunization of human beings against pneumococcus pneumonia is suggested.

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