Active Immunization with Bacteria. 121 



with the different forms of vaccines. For comparative study 

 pneumococci are more satisfactory because of the high virulence 

 of these organisms. Throughout the work type I pneumococcus 

 has been used. 



Four types of vaccine have been studied, (A) plain stock 

 vaccine, killed at 56 0 ; (B) sensitized stock vaccine, killed at 56 0 ; 

 (C) freshly prepared sensitized vaccine, killed at 56 0 ; and (D) an 

 alcohol precipitate of sensitized vaccine similar to that used by 

 Gay in the preparation of typhoid vaccine. 



Immunization was carried out by intraperitoneal injection of 

 increasing quantities at three or four day intervals. Immunity 

 has been studied in mice, guinea pigs and rats. 



Mice immunized with three injections and tested from seven 

 to eleven days after the last immunizing dose, showed immunity 

 from all types of vaccines. The mice immunized with vaccine D 

 were uniformly less immune than those with the other vaccines. 

 There were only slight differences in the animals immunized with 

 plain and sensitized vaccine, and in different experiments the 

 results varied so that neither vaccine can be said to be better. 



Guinea pigs received five immunizing injections in two and a 

 half weeks. In series A there was moderate active immunity; 

 series B very slight; and in series C none. The serum from two 

 animals in each series was tested for antibodies and only in the 

 serum of series A were agglutinins demonstrated. 



Rats were found to be the most satisfactory animals for 

 comparative studies. It was found that in six to ten days after 

 the last immunizing dose, there was a higher degree of immunity 

 in the plain vaccine series, but that this fell off rapidly. On the 

 contrary, in the series immunized with freshly sensitized vaccine, 

 the immunity though present was less marked early but increased 

 after twelve to sixteen days. There was no parallelism between 

 the degree of active immunity and the amount of agglutinin and 

 bacteriotropin in the serum of the immune rats. Agglutinin was 

 demonstrated only in the serum of series A rats. Bacteriotropins 

 were much stronger in the series A rats than in the serum of series 

 C. It is, therefore, evident that animals may possess a high 

 degree of active immunity and still show practically no antibodies 

 in their serum. 



