THE PRODUCTION OF ACTIVE IMMUNITY 163 



and does not become a real active ferment, or it is not 

 activated until the foreign protein comes within its sphere 

 of attraction. It occurred to us that if this theory has 

 much of truth in it we might test it. We thought that the 

 introduction of a small portion of the residue might give 

 some immunity immediately. We therefore injected doses 

 of 25 mg. of the colon and 12.5 mg. of the typhoid residue 

 into the abdominal cavity of guinea-pigs, and thirty min- 

 utes later inoculated these animals intra-abdominally with 

 living cultures, and found that the colon animals had in 

 that short time acquired an immunity of five units and 

 the typhoid one of six units. However, we found that 

 larger immunizing doses did not give us results so good, 

 and this is easily explained by supposing that this ferment 

 set free or activated by the residue is in part used up 

 in its reaction with the residue itself. The reason multiple 

 doses repeated at intervals give us a higher degree of im- 

 munity than single doses may be due to more cells being 

 acted upon or to the accumulation of the ferment in the 

 blood. The theory of the modus operandi of the residue 

 which we have offered is tentative, and we hope to be able 

 to investigate it further. 



It will undoubtedly occur to the reader, as it has to us, 

 to ask the question how it is that the residue sensitizes or 

 activates, while the bacillus itself, living or dead, has no such 

 effect or at least is not nearly so effective. The only answer 

 that we can suggest to this question is that in order to be 

 effective in its action the sensitizer must be in solution, 

 and that being in this state it reaches every part of the 

 circulatory system in a few seconds. Possibly cell permea- 

 tion may be necessary to the most perfect sensitization. 



