THE EXISTENCE OF SENSITIZING SUBSTANCES. 221 



cinatecl against B. pestis contains a sensitizer that endows this bac- 

 terium with the power of absorbing alexin. This sensitizer then acts 

 as do the corresponding substances present in cholera serum and 

 in hemolytic sera. 



It may be added that plague bacilli added to a mixture of alexin 

 and antiplague serum show no morphological alteration after 3 hours 

 at 37 degrees. Consequently in this instance the presence of a 

 sensitizer can be demonstrated only by the fixation of the alexin. 



If in such an experiment much smaller doses of bacilli and anti- 

 plague serum with the same dose of alexin are employed, a complete 

 fixation does not take place. The subsequently introduced cor- 

 puscles are finally hemolyzed, but only after a more or less consider- 

 able delay. 



The serum of guinea-pigs vaccinated with the first anthrax vaccine. 

 — Guinea-pigs were given four successive intraperitoneal injections 

 of the first anthrax vaccine. Five- to six-day pepton-bouillon cul- 

 tures were used for the first two injections. Three- or four-day 

 agar cultures suspended in salt solution were used for the last two 

 injections. 



An experiment similar to the one with antiplague serum was then 

 performed. As a control, in the place of the serum of the vaccinated 

 guinea-pigs, normal guinea-pig serum was used. As a bacterial 

 emulsion a 24-hour agar culture of the first vaccine suspended in 

 salt solution was used. Fresh normal guinea-pig serum furnished 

 the alexin. 



The results were identical with those obtained with antiplague 

 serum. The first vaccine plus normal serum absorbs little or no 

 alexin. There is complete fixation with the specific serum. In 

 this instance, also, the fixation of alexin causes no visible lesion of 

 the bacterium. 



Serum of a horse immunized against swine plague. — Dr. Frasey of 

 the Pasteur Institute was so kind as to furnish us with strongly 

 preventive serum for the swine-plague bacillus. In this serum we 

 were also able by the same method to demonstrate a sensitizer which 

 allows the swine-plague bacillus to fix alexin. 



Serum of guinea-pigs vaccinated against B. typhosus. — These 

 guinea-pigs were given three injections of B. typhosus suspended 

 from agar cultures in salt solution. An experiment modeled after 



