INFECTION AND IMMUNITV. 413 



unwarmed material was used, immunity did not appear 

 until fourteen days, and then only after the employ- 

 ment of relatively large amounts. Moreover, when the 

 previously heated products are introduced into the cir- 

 culation of the animal, the systemic reaction is of but 

 short duration, but if the unwarmed substance is em- 

 ployed, immunity is manifest only after the appearance 

 of considerable elevation of temperature, which lasts for 

 a long time. In explanation of these differences, they 

 suggest that, in the latter case, the high fever that is 

 seen to occur in the animal may serve to replace the 

 warming to which the bacterial products had not pre- 

 viously been subjected, and which is necessary before 

 they are in a position to bring about the condition 

 of immunity. They claim that the bacterial pro- 

 ducts employed in producing immunity in this case 

 are not, in reality, the immunity-affording substance, 

 but that they are only the agents that bring about in 

 the tissues of the animals alterations that result in the 

 production of another body that protects the animal. 

 In support of this, their argument is that several days 

 are necessary for the production of immunity by the 

 introduction into the animal of the bacterial products ; 

 whereas, if the blood-serum of this animal, which is 

 now protected, be introduced into the circulation of 

 another animal, no such delay is seen, but instead, the 

 animal is forthwith protected. In the former case the 

 actual protecting body had first to be manufactured 

 by the tissues ; whereas, in the second it is already 

 prepared, and is introduced as such into the second 

 animal. 



They found the serum of immunified animals to be not 

 only capable of rendering other animals immune, but that 



