﻿STUDIES IN PLAGUE IMMUNITY. 325 



inoculated with the strain "Pest Avirulent" it was found that from six 

 to eight hours after the time of the inoculation the organisms were still 

 very numerous in the tissues, after which time they gradually diminished, 

 and, usually after twenty-four hours, they were no longer reclaimable 

 in cultures. However, in several instances a few colonies developed in 

 the cultures made after this period of time. In immunized animals 

 the organism was destroyed in a shorter period, the cultures made after 

 twelve hours frequently remaining sterile. In these instances smears 

 from the tissues three or four hours after the inoculation showed very 

 extensive phagocytosis of the bacteria. Therefore, the process of immu- 

 nization occurs as in a true vaccination, the organism reproducing itself 

 in the tissues for probably 100 or 200 generations and its successive groups 

 of receptors stimulating the production of corresponding groups of ambo- 

 ceptors in the animal body. It therefore is not difficult to understand 

 why the immunity derived from vaccination in plague is greater than that 

 obtained from the injection of the killed organism. 



Although abundant evidence had been obtained of the immunity pro- 

 duced in animals even more susceptible to plague infection than man 

 (guinea pigs) by vaccination with these attenuated cultures, yet it was 

 interesting to observe what evidence could be discovered, from a study 

 of the blood serum, of the immunity resulting in human beings from 

 such vaccinations. Therefore, the agglutinative reaction of the blood 

 serum of twenty-six of the cases and the anti-infectious power in twenty- 

 four were carefully studied. In order that the experiments might be 

 carefully and completely performed, from 10 to 20 cubic centimeters 

 of blood was collected from a vein, in each case under aseptic precautions ; 

 the blood being drawn ten days after vaccination and the reactions usually 

 performed as soon as the serum had separated from the clot. The results 

 of these experiments have already been recorded during the consideration 

 of the agglutinative and anti-infectious properties of plague serum. The 

 agglutinative reactions are also summarized in Table No. X (p. 252), and 

 the anti-infectious ones in the series of inoculations on pages 275 to 280. 

 Prom these tables it will be seen that the sera possess practically no 

 agglutinating or anti-infectious power, at least in demonstrable amounts. 

 Only in a few instances were there traces of a reaction. However, since 

 in the serum of animals which had been demonstrated to be thoroughly 

 immune to plague these substances (agglutinins and anti-infectious bodies) 

 could not be demonstrated, we should hardly expect to find them in the 

 blood serum of vaccinated human beings. Plague agglutinins and anti- 

 infectious bodies only become developed in demonstrable amounts in 

 animals that have been very highly immunized by large and repeated 

 inoculations of the organism, as has already conclusively been demon- 

 strated. 



The opsonic power of the sera of the vaccinated individuals was next 

 carefully investigated in eight instances, and this reaction proved to 



