NATURAL IMMUNITY 69 



but not by freezing. The blood of different species 

 differs considerably in this regard, and that of the 

 same species may show a decidedly greater germ- 

 icidal action for one disease germ than for others. 

 That the presence of these germicidal substances con- 

 stitutes a most important element in natural immun- 

 ity can scarcely be doubted. According to Behring 

 the blood of the rat and of the frog, which an- 

 imals have a natural immunity against anthrax, is 

 especially fatal to the anthrax bacillus. The numer- 

 ous experiments which have been made show that 

 the germicidal action of blood-serum, which is very 

 promptly manifested, is limited as to the number of 

 bacteria which may be destroyed by a given amount. 

 When the number of bacteria is excessive only a 

 limited number are destroyed, and after an interval 

 those not destroyed multiply abundantly in the blood- 

 serum, which, in the absence of its germicidal con- 

 stituent, is an excellent culture medium for many 

 pathogenic bacteria. 



It would appear from this that the element in the 

 blood to which the germicidal action is due is neu- 

 tralised in exercising this power in other words, 

 that the effect is the result of a chemical reaction. 

 These germicidal substances in the blood of healthy 

 animals are complex nitrogenous compounds, which 

 belong to the group of organic bodies known to 



