54 STl'DIKS IX IMMUNITY. 



for the leucocytes of the mouse, of normal guinea-pigs, or of guinea- 

 pigs vaccinated against cholera. The attraction of the serum of 

 immunized rabbits or guinea-pigs for leucocytes is very much more 

 marked. It will he noted also that cultures of the cholera vibrio 

 suspended in a large amount of salt solution still attract leucocytes 

 strongly. Preventive serum, even when heated for an hour to 

 70 degrees, still attracts leucocytes. 



The leucocytes present in largest numbers in these experiments 

 are the polynuclear amphophiles. 



Are there other points of resemblance between the substances in 

 cultures and those materials characteristic of preventive serum? 

 One of the most noteworthy properties of this serum is that it will 

 immunize with great rapidity even in a small dose. It also may 

 be shown that very small amounts of a sterilized culture of cholera 

 when injected into the peritoneal cavity 24 hours before inoculation 

 of the culture will protect guinea-pigs. For example, 1/200 of a 

 twenty-four hour agar culture of Massaouah was suspended in salt 

 solution, sterilized at 100 degrees, and injected. Controls that had 

 received a corresponding dose of salt solution containing no bac- 

 teria succumbed to infection, whereas the guinea-pigs which had 

 received the tiny doses of culture resisted. An even smaller dose 

 of an older culture was equally successful. When mixed in equal 

 volume with a solution containing 0.6 per cent of NaCl and 0.5 per 

 cent of K,C0 3 and then heated to 100 degrees these small amounts 

 of culture still vaccinate. In the same way serum from immunized 

 rabbits mixed in equal parts with this solution and heated to 100 

 degrees, does not coagulate and preserves for the most part its 

 preventive properties. 



In view of these facts the hypothesis that the preventive prop- 

 erties of the serum are due to a conservation of certain vaccinating 

 substances from the immunizing culture cannot be thrown aside. 

 But if we study the nature of the immunity produced by small in- 

 jections of culture more attentively it appears to be in no respects 

 similar to the immunity afforded by preventive serum. Follow- 

 ing the injection of preventive serum, the serum of the animal 

 inoculated acquires a strong bactericidal power, but no such 

 thing happens after the administration of small amounts of killed 

 vibrio. 



