DISEASE, INFECTION, AND RESISTANCE 287 



contents thoroughly mixed. The content of each tube is 

 then injected into a guinea pig weighing 250 grams. It is 

 probable that the guinea pig receiving the injection of the 

 contents of the first tube will survive, inasmuch as the 

 amount of antitoxin is more than sufficient to neutralize all 

 of the toxin. At the other extreme, however, it is probable 

 that the guinea pig will die, inasmuch as the toxin present 

 is more than completely sufficient to neutralize the anti- 

 toxin. It is evident, then, that the guinea pigs at one end 

 of the series will live and those at the other end will die. 

 Where these two series come together is an approximation 

 of the iieutralization point. The guinea pig, in other words, 

 is used as an indicator in the titration of the toxin against 

 the standard antitoxin. It is necessary arbitrarily to select 

 the point of neutrality. That amount of toxin which when 

 mixed with one immunity unit of the standard antitoxin 

 and injected into a 250 gram guinea pig will kill that guinea 

 pig in just four days is termed an L -(- dose. This amount 

 of the toxic broth is now placed in each of a series of test 

 tubes. That is, each test tube contains one L -|- dose of the 

 now standardized toxin. To the series of tubes is then added 

 graduated amounts of the antitoxin which has been manu- 

 factured and which it is desired to standardize. The contents 

 of these tubes are in turn injected into suitable guinea pigs. 

 Those animals receiving sufficient antitoxin completely to 

 neutralize the toxin injected will live. Those receiving an 

 excess of the toxin will die. Here again an animal may be 

 chosen which dies in just four days. Inasmuch as it received 

 an L + dose of toxin and yet died in just four days it must 

 have also received one immunity unit of antitoxin. 



The entire process may be compared to the standardiza- 

 tion of an unknown alkali by use of a known alkali and 

 unknown acid. The acid is first standardized against the 

 known alkali, and then used for the standardization of the 

 unknown alkali. 



