STUDIES ON THE SERUM OF VACCINATED ANIMALS. 17 



In animals that have been vaccinated by the injection of cultures 

 or by a preventive serum, the chemiotactic response of leucocytes 

 for the bacterium with which the immunization has been brought 

 about has been increased. The number of phagocytes is in general 

 larger in immunized animals than in normal animals : the army of 

 defenders is increased. If phagocytes are filled up by injecting 

 inert powders into the blood stream, or if the animal has been 

 weakened by fasting, cold, bleeding, or over-heating, an infection 

 may develop which under normal conditions might be overcome. 

 The objections that have been directed against the phagocytic 

 theory by partisans of the " bactericidal theory of the body fluids" 

 have been gradually disposed of. The body fluids do not always 

 appear to be of importance in defending the animal body. It has 

 already been noted that bacteria which have been inoculated in 

 sera may suffer simply from the sudden change of medium to which 

 they have been subjected. This is particularly true when they 

 have become unaccustomed to living in body fluids on account of 

 their culture on artificial media. 



Besides, there is no constant or necessary parallel in normal 

 animals, any more than in vaccinated animals, between the anti- 

 septic properties of serum and a refractory state. It is true that the 

 serum of animals vaccinated against the cholera vibrio, or against 

 the vibrio Metchnikovi, is endowed with an energetic bactericidal 

 property against its respective organism, while the serum of normal 

 animals has only very weak destructive properties for these bacteria. 

 But the existence of a strong bactericidal property in the serum of 

 vaccinated animals is exceptional rather than usual. Animals well 

 immunized against pneumonia, tetanus, hog-cholera, diphtheria, 

 etc., have sera endowed with preventive or curative properties, 

 which show no bactericidal property. The destruction of bacteria 

 in such animals is always brought about by phagocytes. 



The theory of the " attenuation of bacteria by the body fluids" 

 is no more general in application than is the theory of the "bac- 

 tericidal property of the body fluids." Organisms cultivated in 

 the serum of vaccinated animals generally become accustomed to 

 this medium and suffer no diminution in their pathogenic power. 

 No diminution, moreover, is produced by passage through a vacci- 

 nated animal; on the contrary, it is frequently to be noted under 



