214 APPLIED IMMUNOLOGY 



tericidal substances, (a) leucocytic bacteriocidin, (b) 

 humoral bacteriocidin, and that certain varieties of 

 bacteria are destroyed by the one, and other varieties 

 by the other. These two types of bactericidal sub- 

 stances are found in the fluid part of uncoagulated 

 blood, that is, in the plasma. If, however, the blood 

 is allowed to coagulate, or is defibrinated, only the 

 humoral substance is found in the serum. From this 

 it is concluded that one bactericidal substance is de- 

 rived from the leucocytes, and is probably identical 

 with fibrinogen, while the serum or humoral bacterio- 

 cidin does not come from the leucocytes. 



It has been found that streptococci and pneumo- 

 cocci are killed only by the blood-plasma, i,e,, by the 

 leucocytic substances; these organisms multiply in 

 serum alone. On the other hand, typhoid bacilli are 

 destroyed by serum or humoral bacteriocidins. In this 

 way is explained the clinical fact that in streptococcic 

 and allied infections there is a marked leucocytosis, 

 while in typhoid fever the leucocyte count remains 

 normal. 



It has further been found that the humoral bac- 

 tericidal substances lose their activity by heating the 

 serum to 55^ C, but that they can be reactivated by the 

 addition of fresh normal serum; in other words, their 

 action depends on the presence of complement. On the 

 other hand, the leucocytic substances are not inacti- 



