38 STUDIES l\ IMMUNITY. 



of the most interesting of those- changes, as we have already observed, 

 is the ease with which certain bacteria gain the power of absorbing 

 eosin instead of methylene blue. On account of the regularity with 

 which this phenomenon occurs, one is inclined to attribute the 

 pseudo-eosinophilic granules in macrophages to bacterial origin. 

 As for the morphological changes in engulfed bacteria, it is to be 

 noted that there are present, in leucocytes of normal animals that 

 have received an injection of cholera vibrios, the same sort of 

 granules (transformed vibrios) as in the peritoneal fluid of highly 

 vaccinated animals. In the normal animals, however, the bacte- 

 ricidal property, as evidenced by the transformation of the vibrio 

 into granules, is never so energetic, and brings about the change 

 only where the bactericidal substance is highly concentrated, that 

 is to say, within the phagocytes. 



IV. Leucocytes and the Preventive Power of Serum. 



It is well known that the blood of animals that have been vacci- 

 nated several times with a culture of living vibrios killed by heat, 

 has not only bactericidal but preventive properties. The injection 

 of serum from such vaccinated animals into healthy animals per- 

 mits the latter to withstand a dose of culture that, under ordinary 

 conditions, is certainly fatal. There are preventive cholera sera 

 which are so very powerful that they act in doses as small as two 

 or three milligrams. 



Will facts offered in explanation of the bactericidal property of 

 serum also explain the preventive power? Are the substances that 

 endow the serum of vaccinated animals with their preventive 

 properties present in the body fluids of the living animal? Or are 

 they, on the contrary, more or less retained by the white blood 

 corpuscles during life? The experimental procedures already 

 described are also applicable to this problem. 



It is perfectly evident that the relative preventive value of serum 

 and of edema fluid from the same vaccinated animal may be com- 

 pared. We may also compare two specimens of blood from the 

 same animal containing different numbers of leucocytes, and this 

 method we have employed. In these experiments we used the 

 vibrio of Massaouah, which is an advantageous culture for two 

 reasons : its virulence is relatively constant, and, secondly, it is so 



