102 STUDIES IN IMMUNITY. 



To be sure Gruber does not believe that union takes place within 

 the protoplasm of the phagocyte, simply because he does not believe 

 that the phagocyte is the source of the normal bactericidal sub- 

 stance. He does not bring any new facts to bear on this discussion, 

 but simply accepts Pfeiffer's opinion which we have already con- 

 sidered. 



The task of explaining the intimate changes which take place 

 in bacteria as a result of the activity of the serum (immobilization, 

 clumping, granular transformation) is far from finished. Gruber, 

 to be sure, has offered an explanation of clumping, but, as we have 

 already seen, it is far from satisfactory. 



Pfeiffer has a different point of view on the action of sera. He 

 thinks that the immunizing substances in immune serum are present 

 in these sera in an inactive form, and become active by undergoing 

 certain changes within the animal body. 



In the first place it is certain that sera contain active substances, 

 since they can produce the same modifications on bacteria in vitro 

 as in vivo. These modifications, moreover, are exactly alike, and 

 equal whether in vivo or in vitro. And, further, it is easy to show 

 that the preventive substance injected is simply diluted in the 

 blood of the animal and does not become more active. 



The serum of a guinea-pig vaccinated against the cholera vibrio 

 acquires a property that it did not previously have, namely, the 

 property of clumping the cholera vibrio. We know, moreover, that 

 the better a serum clumps the more actively preventive it is. The 

 preventive value, then, may be measured by the clumping property. 

 If we place in separate tubes a given amount of an emulsion of 

 vibrios and add to each tube a varying number of drops of pre- 

 ventive serum we can determine exactly what this clumping power 

 is. Controls, of course, are made with emulsion without preventive 

 serum. After a certain time the deposition of bacteria and the 

 clarification of the supernatant fluid reaches its maximum (about 

 24 hours when the doses of serum are small). In this way the 

 smallest amount of serum which will cause either a beginning of 

 clarification or complete limpidity may be determined. 



A guinea-pig weighing 300 grams was bled and then injected with 

 1 c.c. of active preventive serum; the next day the animal was 

 again bled. The serum obtained before injection had only a 



