THE EXISTENCE OF SENSITIZING SUBSTANCES. 219 



treated with a hemolytic serum heated to 55 degrees) are added to 

 fresh guinea-pig serum containing alexin, their dissolution follows. 

 If after a certain time sensitized cholera vibrios (i.e., vibrios treated 

 with heated cholera serum) are added and the mixture placed in the 

 incubator, no transformation or change takes place in the vibrios. 

 From this result we know that there was no free alexin in the mix- 

 ture, for, if present, it would have transformed the vibrios. From a 

 control tube we learn that a transformation occurs in the vibrios if 

 the red blood corpuscles added in the first place are not sensitized. 



The converse of this experiment also holds. If sensitized cholera 

 vibrios are added to normal alexic serum, subsequently added sen- 

 sitized corpuscles are not hemolyzed. 



From such experiments we drew, it will be recalled, two distinct 

 conclusions : First, corpuscles or bacteria when sensitized are able to 

 absorb alexin with avidity and to remove it from the surrounding 

 fluid; second, in a given serum the same alexin may produce either 

 hemolysis or bacteriolysis.* 



These conclusions find still further confirmation in the following 

 article. In the present article a fact suggested by the experiment 

 just outlined is of preeminent interest: To demonstrate the existence 

 of a sensitizer in an antimicrobial serum we may make use of its 

 property of causing the bacterium it affects to absorb alexin. 



As an experiment to demonstrate this fact is practically the same 

 with any one of the antimicrobial sera studied, it may be described 

 in detail once for all. We shall take as an example antiplague 

 serum. 



Serum of a horse vaccinated against B. pestis. This strongly pre- 

 ventive serum was kindly furnished us by Dr. Dujardin-Beaumetz, 

 who has charge of preparing it and testing its potency at the Pasteur 

 Institute. 



This serum and normal horse serum were heated to 56 C for one 

 half hour to destroy the alexin. A 24-hour agar culture of B. pestis 

 was suspended in a small amount of salt solution so as to form a 

 thick emulsion of bacteria. Fresh guinea-pig serum obtained 

 by bleeding the guinea-pig the day before, and freed from cor- 

 puscles by centrifugalization, was also at hand and was used for 

 alexin. 



* The unity of the alexin in a given serum is also admitted by Buchner. 



