IMMUNIZATION AGAINST DIPHTHERIA. 483 



about 10 times the minimal dose mixed with iodine should 

 be administered. 



The animals have now acquired a certain resistance to the 

 toxin and the addition of iodine should be omitted. The 

 dose of pure toxin should be small for the first injection, 

 preferably the minimum fatal dose. Several injections of 

 this amount should be given and later the dose may be 

 doubled or quadrupled. The injections are repeated with 

 the same or gradually increasing doses as the health of the 

 animals will permit. The best results in immunization are 

 always obtained by proceeding slowly. The animals should 

 be given plenty of time to recover from the ill effects of the 

 last injection. 



Eventually, large doses of toxin can be tolerated by the 

 experimental animals. The fact that a condition of immun- 

 ity exists can be easily demonstrated by injecting a new 

 untreated rabbit with the same amount of toxin as is given 

 to the treated animals. 



When the animals have been immunized to such an ex- 

 tent that they can bear a large dose of toxin, say 50 or 100 

 times the minimum fatal dose, a small amount of blood may 

 be drawn and its antitoxic properties tested. The best 

 procedure, in this case, will be to draw about 3 c.c. of blood 

 from the jugular vein by means of a syringe (p. 461), and to 

 inject this at once, subcutaneously, into a guinea-pig. The 

 latter, as well as a control animal should then be given an 

 injection of 5 or 10 times the minimum fatal dose. The ani- 

 mal should survive, and, if the amount of antitoxin is suffi- 

 cient, it will show no induration, swelling or scar at the 

 point of inoculation. 



The injections should always be made subcutaneously. 

 The toxin should be made up to a convenient volume (1 or 

 2 c.c.) by the addition of physiological salt solution. It is 

 hardly necessary to add that all the instruments and glass- 

 ware employed must be sterile. 



