244 STUDIES IN IMMUNITY. 



other words, if, instead of the mixture of alexin, corpuscles and 

 hemolytic serum (55 degrees), a mixture is prepared in which the 

 specific hemolytic serum is replaced by normal serum from the 

 same species, the fluid remains full of alexin and retains its de- 

 structive power for sensitized cells. Normal sera, then, as a rule 

 are unable to produce alexin absorption. If normal serum con- 

 tains sensitizers, they are either too little in amount or too 

 inactive to cause a fixation of alexin as determined by the method 

 described. 



There are, however, certain exceptions to this general rule. 

 There are normal sera the alexin of which may be fixed by cer- 

 tain cells without the presence of an immune serum. Malvoz* 

 has recently shown that this is the case with dog serum mixed 

 with B. anthracis; this serum, moreover, in presence of this organ- 

 ism will cause the fixation of the alexins of other sera (rabbit, 

 guinea-pig, rat) ; in other words, it acts as if it contained a true 

 sensitizer. 



In a similar way Bordet and I have recently found that, in a 

 mixture of fresh normal dog serum and washed rabbit corpuscles, 

 hemolysis occurs rapidly, and a fixation of alexin takes place to 

 such an extent that subsequently added sensitized vibrios undergo 

 no transformation. 



Such cases, however, are the exception. The normal serum of the 

 majority of laboratory animals does not affect cells enough to 

 cause them to fix alexin to an appreciable extent ; immune sera, on 

 the contrary, render cells very avid of this substance. The reaction 

 of alexin fixation in general shows a distinction between an immu- 

 nized and a normal animal of the same species. In other words, it 

 demonstrates the presence of the specific sensitizers produced by 

 vaccination. 



Previous to Bordet's experiments on alexin absorption a sensi- 

 tizer could be defined simply as a substance that renders a given 

 cell destructible, or at least morphologically alterable, by a dose 

 of alexin that normally has no effect on it. But since the sensitizer 

 has not only the property of increasing the harmful effect of the 

 alexin, but also the property of causing the cell it affects to absorb 

 * Annales de 1'Institut Pasteur, August, 1902. 



