100 STUDIES IN IMMUNITY. 



ever, as Issaeff (Annales Pasteur, 1893) has shown are still very 

 virulent. 



As a general thing cultures of pathogenic organisms grown in the 

 serum of immunized animals remain virulent whether the organisms 

 are clumped or not (the pneumococcus on the one hand and the bacil- 

 lus of hog-cholera described by Metchnikoff in 1892 on the other). 



Horse serum and even horse edema fluid has a remarkable clump- 

 ing effect on the cholera vibrio. The struggle against the vibrios, 

 however, in the body of this animal takes place just as it does in other 

 animals, namely, by means of phagocytosis. If we inject an agar cul- 

 ture of a virulent cholera vibrio subcutaneously in a horse,* an influx 

 of leucocytes occurs. A purulent fluid may be removed the next 

 day from the area of inoculation and is found to contain leucocytes, 

 in the protoplasm of which are found the organisms in various 

 stages of degeneration; there are no free bacteria present. The 

 inoculation of this pus on agar even 30 hours after injection gives 

 an abundant culture. In other words, the bacteria have been 

 taken up alive and eventually die within the phagocytes, as is 

 shown by the fact that the animal rapidly recovers. 



Does this mean that the extracellular changes in bacteria, the 

 most important of which is granular transformation, is without 

 importance in immunity? In no manner; by this means the num- 

 ber of invaders may be decreased, their development hindered and 

 time given the animal to combat them advantageously by means 

 of the leucocytes, that have eventually come up, instead of being 

 faced by an invincible number of adversaries and their elaborated 

 toxic substances. 



In order to explain the mechanism of passive immunity we must 

 account for one very important property acquired by animals 

 treated with preventive serum. To what is the bactericidal power 

 that is given these animals due? We have nothing essentially new 

 to add to observations already made on this subject. The mixture 

 in vitro of fresh normal serum and a small amount of preventive 

 serum, neither of which sera is in itself bactericidal, produces a 

 fluid that is strongly bactericidal. We have repeatedly emphasized 

 this experiment because it appears to us to be the explanation of 

 the appearance of a bactericidal property in the blood of passively 



* The culture was kindly furnished us by Dr. Salimbeni. 



