NATURAL RESISTANCE 169 



a slight degree, and to recover from it rapidly. In 

 some instances, the immunized animals were able to 

 withstand the intravenous injection of a quantity of 

 cytost from 100 to 500 times as great as the amount 

 found to cause the death of normal animals within 

 three minutes. Furthermore, the immunized animals 

 proved to be refractory to the usual procedures which 

 are known to induce shock. (Turck, 1918, 1921.) 



These striking results prove that the preliminary 

 series of cytost injections cause the elaboration of 

 some defense mechanism within the body of the 

 treated animals. These facts, taken in conjunction 

 with our previous observation that cytost is to a 

 marked degree species-specific, suggest that the in- 

 creased tolerance to cytost shown by the treated ani- 

 mals is due to the elaboration of a specific antibody or 

 anticytost. 



Two different sets of experiments tend to substan- 

 tiate this concept. First, it was found that cats which 

 had been subjected to a series of injections of heterol- 

 ogous cytost prepared from beef or rat tissues did not 

 develop an increased resistance to cytost. This was 

 demonstrated by the fact that animals so treated, when 

 injected with homologous cytost passed into shock 

 and died as rapidly as did untreated normal animals. 



These results offer further evidence that the proc- 

 ess of active immunization to cytost depends upon the 

 elaboration of a specific anticytost analogous in some 

 respects to the specific antibodies of classical im- 

 munology. By analogy, then, it appeared likely that 

 such antibodies might be produced in the body of 

 another animal, such as the horse or goat, and that pos- 

 sibly by injection of the serum of such an immunized 



