164 STUDIES IN IMMUNITY. 



stances for the purpose of defending itself, but simply puts into 

 action preexisting functional capacities that, under proper condi- 

 tions, might act on such innocuous substances as red blood cells or 

 milk casein, as well as bacteria. The special properties that are 

 found in the sera of vaccinated animals are present in a primitive 

 form in normal sera. This fact probably has a distinct bearing on 

 the specific nature of these substances in immune sera. 



