250 INFECTION AND IMMUNITY. 



amboceptor, is heat-resistant (thermostabile), i. e., 

 it is not destroyed at 56 C., whereas the other, 

 the complement, is susceptible to heat ( therm o- 

 labile), being destroyed at that temperature which 

 killed the alexins of Buchner. The term alexin 

 is still applied by some writers to the thermolabile 

 substance (complement), its original significance 

 having been modified. 



specificity. The specificity which prevails among antitoxins 

 and agglutinins is found also in the action of bac- 

 tericidal serums. When an anticholera serum is 

 injected into the peritoneal cavity of a guinea- 

 pig, protection is not afforded against other vibrios 

 or other pathogenic organisms. The specificity is 

 so great that the reaction of Pfeiffer may be used 

 for the identification of bacteria. If one has in 

 hand an unknown vibrio, its identity or non-iden- 

 tity as the organism of cholera may be determined 

 by injecting it, in conjunction with anticholera 

 serum, into the peritoneal cavity of a normal 

 guinea-pig; if the microbe is transformed into 

 granules it is the vibrio of cholera, otherwise it is 

 not. Other bacteria may be identified in a similar 

 manner by the use of the proper serums. In spite 

 of this high specificity, the group reaction may 

 Group Reac- occur even with bactericidal serums. An anti- 

 typhoid serum, for example, shows its strongest 

 bactericidal power for the typhoid bacillus, al- 

 though it is at the same time more destructive for 

 closely related organisms, as the colon bacillus, 

 than a normal serum from the same species. By 

 diluting the serum sufficiently the adventitious 

 bacteriolysins are so nearly eliminated that the 

 specificity of the serum for its homologous organ- 

 ism becomes manifest. 



