144 COLLECTED STUDIES IN IMMUNITY. 



Two rabbits were inoculated intravenously, one with 



3 units serum + 0.3 cc. toxin, 



a mixture neutralized about three fold ; the other with 

 3 units serum + 0.45 cc. toxin, 



a mixture about doubly neutralized. 



The animals received daily increasing doses from the 5th to the 

 18th of December, 1900, at the end of which time the total amount 

 of toxin they had received was 



for the one 7.5 cc. or 750 fatal doses, 

 for the other 4.27 cc. or 429 fatal doses. 



The animals showed no change in health and lost no weight. 



In order to allow the excess of serum introduced time to be 

 eliminated, four weeks were allowed to elapse before testing the serum 

 for its antitoxic strength. 



A control rabbit treated with serum alone died accidentally, but, 

 as will be seen from the results of the experiment, a control was 

 superfluous. 



Both rabbits were killed Jan. 24, 1901, and 1 cc. of the serum 

 was mixed with one-quarter a Lt dose. The test animals died in 

 twenty-four hours. By decreasing the quantity of toxin to one- 

 eighth L-J- dose, death occurred in forty eight-hours. 



From this we see that the serum of these animals certainly con- 

 tains no more than one-eighth of an immunizing unit, an amount 

 which at once eliminates any idea of a passive immunity. 



One must therefore conclude that the organism of a normal 

 rabbit not sensitized through previous immunization is unable to break 

 up the combination of diphtheria toxin with antitoxin. Not a trace of 

 this toxin is free at any moment, and the strongest doses of the mix- 

 ture are destitute of any injurious effects. Twenty fatal doses, for 

 instance, were given at the beginning. But we see further that these 

 mixtures do not cause even the slightest production of antitoxin. 

 We must therefore conclude, with Arloing, that the injection of 

 over-neutralized toxin is absolutely useless for purposes of immuni- 

 zation. 



These results do not in the least resemble those of other authors 

 who have used partially neutralized mixtures in which toxons and 

 toxonoids are present in a free state. So far as immunizing power 



