ON THE MODE OF ACTION OF PREVENTIVE SERA. 101 



immunized animals. The study of an infection in such passively 

 immunized animals shows that their resistance is due to a mechan- 

 ism similar to that which we described in June, 1S95 as occurring in 

 vitro. Bactericidal power arises in the animal body, just as it does 

 in a test tube, by the union of two substances: a specific preventive 

 substance, and the normal bactericidal substance present in the 

 blood, not only of immunized, but also of normal animals. Sepa- 

 rately, each one of these substances has only the slightest effect; 

 together, they change the micro-organism most evidently. 



The normal animal has the normal bactericidal substance. By 

 treating this animal with immune serum we give to it the preventive 

 substance which is lacking. How do these two substances unite? 

 If the bactericidal substance under normal conditions is uniformly 

 dissolved in the body fluids, the union would take place in these 

 fluids. But we know that this bactericidal substance is concen- 

 trated in the leucocytes. The antiseptic mixture, then, must be 

 formed primarily by a union within the leucocytes and can take 

 place outside the cells only when a certain amount of the alexin has 

 been liberated by the cells into the surrounding fluid. 



Passive immunity is due then, at least partially, to the chemical 

 effect of two preexisting substances on the vibrios. One of these 

 substances is present in the animal before injection and the other 

 comes from the serum injected. The phenomenon is chemical 

 in the sense that it can occur without the aid of any vital reaction 

 or any new cellular secretion, as we know from the fact that it 

 occurs in fluids that contain no cells. With the aid of this new 

 means of defense, which has been acquired without reaction, the 

 leucocytes are more than ever able to utilize their protecting power.* 

 Passive immunity then may be explained, at least in part, as a pas- 

 sive increase of the phagocytic bactericidal power. Gruber tacitly 

 agrees to this explanation, at least so far as the production of a 

 bactericidal property by means of the union of two substances is 

 concerned. It is, moreover, evident that he does accept this explana- 

 tion, since he has drawn similar conclusions in his recent articles. 



* We do not wish to imply that the leucocyte does not react at all to this 

 injected serum, although a reaction by the leucocyte is not indispensable for the 

 genesis of bactericidal power. There is no reason to suppose that the leucocyte 

 may not react by an acceleration of its phagocytic activities. 



