THE PRODUCTION OF ACTIVE IMMUNITY 147 



For example, No. 3, which received a total amount of 0.3 

 gram in nine doses, was able to withstand 2 c.c. of a sixteen- 

 hour culture after an interval of sixteen days, while No. 4, 

 which received the same total amount in two doses, suc- 

 cumbed to the injection of 2 c.c. of a sixteen-hour culture 

 given at an interval of four days after the last dose of 

 residue. Again we notice that the length of time over which 

 the immunity lasts is rather short in the case of animals 

 which have received a large amount of the substance in a 

 few doses continued over a short period. Thus the immunity 

 to 3 c.c. of a culture, 0.5 c.c. of which proved fatal for 

 untreated pigs, was lost between the fourteenth and the 

 seventeenth day following the injection of the last dose of 

 residue, and that to 4 c.c. of the same culture disappeared 

 between the seventh and the fourteenth day. 



That it is possible to secure active immunity to the living 

 colon bacillus in rabbits by the injection of the colon residue 

 is shown in the following experiments: 



Rabbit No. 1 received on August 13 a solution which 

 contained 1 gram of the residue. On August 20 a second 

 injection of 2 grams was given. Eighteen days later this 

 animal was inoculated with 5 c.c. of an eighteen-hour culture 

 of the colon bacillus. From two to four hours after injec- 

 tion this rabbit was apparently very sick, the symptoms 

 resembling those which are seen following the injection 

 of the toxic portion. However, it then began to improve, 

 and eventually completely recovered. A control inoculated 

 at the same time with an equal amount of the same culture 

 died in five hours. 



Rabbit No. 2 received doses of 1 gram of the residue on 

 October 5, 10, and 20. Four days after the last injection 

 this animal was inoculated with 5 c.c. of an eighteen-hour 

 culture from which he recovered. A control given the 

 same dose at the same time died within five hours. 



Rabbit No. 3 had the same treatment as did No. 2, and 

 six days after the last dose of residue withstood 5 c.c. of 

 an eighteen-hour culture. Eight days later this animal 

 received 10 c.c. of a sixteen-hour culture and did not die. 



