86 STUDIES IN IMMUNITY. 



organisms grow out again quite as well as when the phenomenon 

 has been produced in a serum mixture in vitro.* 



Pfeiffer's objection, then, is untenable. As a matter of fact, the 

 power of transforming vibrios is very marked in immune serum. 

 There is nothing astonishing in the fact that after a certain time 

 the bactericidal properties are exhausted and, with in-vitro experi- 

 ments when the substance is used up, there is no means of renewing 

 it. In the animal body, however, the exudate keeps on increasing 

 after injection. Moreover, it would not be surprising to find that 

 the exudate is more bactericidal than serum, since even at the 

 moment of injection it often contains more leucocytes than does the 

 blood. And finally, there is every reason to believe that the extent 

 of the humoral bactericidal activity which occurs in the peritoneal 

 cavity of immunized animals is exaggerated. Certain vibrios resist, 

 and either retain or recover their motility; and, under the usual 

 conditions of Pfeiffer's experiment, it is not reasonable to speak of 

 an extracellular destruction that occurs 3 or 4 hours after injection 

 for the simple reason that by this time all the organisms, whether 

 altered or not, are within leucocytes. After this time any further 

 destruction of the infective agent must be attributed, not to the 

 fluid, but to the cells. Pfeiffer has repeated Metchnikoff's experi- 

 ment of injecting into a normal guinea-pig a mixture of vibrios and 

 preventive serum in an area where edema has been caused by venous 

 compression. Under these conditions Metchnikoff found that the 

 vibrios outside the cells were not changed into granules. In this 

 experiment naturally a preventive serum that has not been freshly 

 obtained must be used, since a fresh serum might of its own accord 

 produce a metamorphosis of the vibrio. Pfeiffer obtained a different 

 result from the one noted by Metchnikoff, that is to say, he did find 

 an extensive extracellular change in the organisms. Metchnikoff's 

 results, however, were constant. We have found, moreover, that in 

 vitro the cholera vibrio is unchanged by a mixture of preventive 

 serum and edema fluid, whereas it is transformed by a mixture of 

 preventive serum and fresh normal serum. The unexpected result 

 of Pfeiffer's experiment may be due to the fact that this edema was 

 not pure plasma, but contained a certain amount of blood. The 

 punctures that have to be made to obtain the exudate may very 



* This was very clearly shown in Metchnikoff's experiments. 



