36 . APPLIED IMMUNOLOGY 



First of all it is necessary to obtain a quantity of 

 the toxin, separated from the bacteria. The diph- 

 theria bacilli are grown in bouillon, and the fluid con- 

 taining the toxin freed from bacteria by passage 

 through a Berkefeld filter. The toxin strength of the 

 fluid must then be measured. The unit of toxin 

 strength is the smallest quantity necessary to kill a 

 guinea-pig weighing 250 grammes in 5 days or less. 

 This quantity of toxin is called the minimal lethal dose. 

 It should be noted that not all strains of diphtheria 

 bacilli form toxin suitable for the production of anti- 

 toxin. For the formation of antitoxic serum, the 

 horse has been found to be the most suitable animal, 

 but individual horses difl*er in the quantity and quality 

 of antitoxin they are able to produce. No definite 

 rules can be laid down for the active immunization of 

 the horse against diphtheria toxin. But in a general 

 way, small doses of the toxin are first injected, and 

 gradually increased, until a very large quantity of the 

 toxin can be injected at one time. In regulating the 

 amount of toxin to be injected, one is guided by the 

 effects of the previous dose, which are not shown im- 

 mediately as a rule, but appear after a few days. 



From time to time, small quantities of blood are re- 

 moved from the jugular vein of the horse, and tested 

 for antitoxic strength. When a sufficiently powerful 

 antitoxic content has been demonstrated in the horse's 



