STUDIES ON THE SERUM OF VACCINATED ANIMALS. 51 



this difference. There is distinct correlation between the preventive 

 substances and the bactericidal substances employed during the 

 process of immunization. It seems hard to realize a priori how an 

 animal can be so delicately constructed from the chemical stand- 

 point that according to necessity it may build up from its own 

 elements substances which are preventive, now against one vibrio 

 and now against another. It seems, therefore, quite reasonable that 

 the preventive substance may be a simple bactericidal substance 

 more or less transformed and changed. This was the hypothesis 

 that Buchner offered in such infections as diphtheria and tetanus, on 

 the ground of certain analogies between the properties of diphtheria 

 toxin and antitoxin. Buchner thought that antitoxin was derived 

 from toxin since the preventive power of a serum depends on the 

 amount of toxin that has been injected rather than on the real 

 resistance of the animal.* 



We do not know the nature of the preventive substance in cholera 

 serum and we have no knowledge concerning the toxin this 

 micro-organism forms. We do not know whether the substance 

 that gives rise in the immunized animal to the preventive substance 

 is the toxin or some other vaccinating substance in the culture. In 

 the extremely vague position in which we are at the present time it 

 is best to review the various properties which the preventive sera 

 and the bacterial products are known to possess, and to note what- 

 ever conditions and analogies occur in comparing these properties. 



The culture products from the cholera vibrio have an attraction 

 for leucocytes. Does preventive serum have the same property? 

 And does it differ from normal serum in this respect? 



We already know that inoculation of preventive serum into the 

 peritoneal cavity causes an increase in the number of leucocytes 

 there. It may certainly be concluded from this fact that the influx 

 of leucocytes is due to some attraction on the part of the serum. 

 Chemiotaxis, however, is not the sole influence to which the influx 

 of leucocytes is due. It is quite possible that it may be one of the 

 causes that produces this collection of leucocytes, but there is cer- 



* This fact has been established not only in tetanus and diphtheria, where it is 

 a question of toxin and antitoxin, but also in instances where this combination 

 does not occur, as, for example, in the serum of animals vaccinated against the 

 hog-cholera bacillus studied by Metchnikoff. 



