ON THE MODE OF ACTION OF PREVENTIVE SERA. 99 



III. The Function of Cells and of Body Fluids in Immunity. 

 The Mechanism of Passive Immunity and its Rela- 

 tions to Active Immunity. 



In the first section of this article we recalled to the reader experi- 

 ments that demonstrated the respective activities of the cells and 

 the body fluids on vibrios that had got into the tissues. It was 

 mentioned that cells may act either by liberating bactericidal proper- 

 ties in the surrounding fluid or by taking up intact or modified bac- 

 teria. It was evident also that of these two modes of action the 

 second is the more important and the only essential one. Phago- 

 cytosis takes place in all cases and neither previous granular 

 change nor loss of motility are necessary for its occurrence. The 

 body fluids in transforming or changing vibrios act simply because 

 they have received active principles from the leucocytes. Although 

 it is true that the fluids may in certain cases destroy large numbers 

 of vibrios, phagocytosis nevertheless occurs markedly and is the 

 ultimate means of destroying the infection. 



Neither the clumping nor immobilizing of bacteria is to be con- 

 sidered as indicative of their destruction or as an indication of 

 their elimination without phagocytosis, even to the extent that 

 granular transformation is. We have already shown that clump- 

 ing of bacteria is no indication of their alteration, that their sur- 

 face has become viscous, nor that they are swollen or incapable of 

 reproduction. Moreover, this phenomenon is never complete in 

 the animal body and, when the transformation of vibrios into 

 rounded bodies follows in the peritoneal cavity, many granules 

 are seen that remain separate. Moreover, the clumping power of 

 a given serum bears no relation to the resistance of the animal that 

 furnished the serum to the bacterium in question. Horse serum 

 clumps tetanus bacilli energetically, but the horse is very suscep- 

 tible to tetanus; much more so, indeed, than is the rabbit, and yet 

 the serum of this latter animal has no effect on the tetanus bacillus. 

 The rat is refractory to cholera, but its serum has no effect on the 

 cholera vibrio. 



An organism may show most evident clumping and yet retain 

 its virulence completely. The pneumococcus when grown on the 

 serum of immunized animals forms clots; such cultures, how- 



