STUDIES IN I MM I XITY. 



the others Bhowed that the active scrum of these guinea-pigs causes 

 an energetic fixation of alexin by B. typhosus. 



It seemed worth while in this instance to determine whether this 

 sensitizer, that causes a fixation of alexin, is strictly specific. For 

 this purpose the experimental tubes were doubled in the following 

 manner: One series of tubes contained mixtures of normal and 

 specific sera with or without the emulsion of specific bacilli, as has 

 already been described. In a second series of tubes the amounts of 

 serum were identical with the first series, but an emulsion of B. 

 coli instead of B. typhosus was used. Twenty-four-hour agar 

 cultures of each organism suspended each in 4 c.c. of salt solution 

 were used. The agar surface covered was approximately the same 

 with each organism; and yet the colon suspension, judging from its 

 growth, contained more organisms. We mention this point so that 

 a failure of B. coli to fix the alexin in presence of antityphoid serum 

 may not be attributed to an insufficient dose of bacteria. 



The result of such an experiment is very clear. The antityphoid 

 serum shows very marked and yet not absolute specificity. In 

 fact the colon bacillus when mixed with antityphoid serum acquires 

 the property of absorbing alexin to a certain degree. Whereas 

 relatively small doses of the typhoid bacillus and antityphoid serum 

 absorb alexin completely, much larger doses of B. coli with the 

 same serum are required to produce even a partial fixation. For 

 example, if sensitized blood corpuscles are added to a mixture of 

 0.2c.c. of guinea-pig alexin, 0.2c. c. typhoid emulsion and 0.6c. c. anti- 

 typhoid serum (56 degrees) that has been standing for a few hours, 

 they remain indefinitely intact; in a mixture containing the same 

 amount of alexin and twice the dose of antityphoid serum and 

 colon emulsion, the same dose of corpuscles is hemolyzed after an 

 hour's delay. There is a very distinct delay in this tube, as the con- 

 trols containing bacteria are completely hemolyzed in 15 minutes, 

 and those without bacteria in 2 minutes. 



It is evident, then, that the colon bacillus reacts distinctly, though 

 much less powerfully, than the typhoid bacillus to the sensitizing 

 effect of antityphoid serum* 



Serum from convalescent typhoid patients. — Dr. Widal has been 



* It may be noted that this serum agglutinates B. typhosus well, but has no 

 more effect on B. coli than normal serum. 



