Leucocyte Counts and Bacteremia Studies in Rats Orally Immunized Against Pneumococci

Various experimenters 1 have shown that young rats can be vaccinated per os against multiple lethal doses of virulent pneumococci. Ross has shown that the blood serum of such orally immunized animals contains antibodies which may be demonstrated by means of the passive immunity conferred upon other animals to which it is transferred. Besides this indication of a general humoral resistance to infection, we have seen no attempts at explanation of the mechanism of the immunity of such animals, and it was thought that studies of the leucocyte response, together with blood culture examinations, might throw some light upon the subject.

The method consisted of making periodic tests of the peripheral Wood of a series of normal rats and comparing the results with those of a series of rats previously immunized with egg-white and Type I pneumococcus autolysate, as already described. 2 Both sets of rats were inoculated intraperitoneally with 100 fatal doses of the homologous strain of pneumococci. Blood was obtained under sterile precautions from the tail of the animal; examinations were continued as long as the unvaccinated rats survived (2 to 4 days).

From work done on normal persons, it was expected that the leucocyte counts would be subject to wide fluctuations. This proved to be the case. One feature of the leucocyte counts, however, was constant; the immunized animals, after the first 6 hours following injection, showed an appreciably higher white cell count than the unvaccinated controls. This may be attributed to increased mobilization of leucocytes without any considerable diapedesis into the peritoneum. The lower counts in the unvaccinated animals probably reflect the loss of leucocytes which go to make up the exudate in the peritoneal cavity. This loss may go on until a leucopenia occurs, especially if the rats do not die until the third or fourth day.