The data derived from the above described investigations indicate that dogs do not develop hypersensitivity to the pneumococcus as the result of experimental lobar pneumonia. This inference is based on the following findings:
1. Fifteen dogs were given Type I and Type II pneumococcus lobar pneumonia and following recovery were tested for hypersensitiveness by means of intrabronchial and intracutaneous injections of the autolysate made from the homologous pneumococcus.
2. Seven dogs showed a pulmonary lesion discernible with the X-ray at site of the autolysate inoculation; three of these dogs were normal controls.
3. No evidence of a positive skin reaction was found in any of the fifteen dogs, many of which received repeated infections and intradermal autolysate injections.
4. Subsequent infections in the same animals were definitely milder than the initial infection.
5. The infections following the administration of intrapulmonary and cutaneous autolysate were practically of the same intensity as the initial infection.
6. Temperature, pulse rates, white blood counts and differential blood pictures showed no significant variations following intrapulmonary injection of autolysate.
7. Tests for the acquisition of humoral immune bodies following autolysate injection and recovery from the experimental disease showed the presence of these substances in some of the dogs and their absence in others.
8. Study of the pathology of the pulmonary lesions produced by the autolysate failed to reveal histological changes characteristic of an allergic reaction. However, the presence of perivascular accumulations of large mononuclear cells observed in the lesions of the recovered dogs does suggest a locally accelerated reactivity of the fixed tissue cells to the products of the pneumococcus.