206 STUDIES IN IMMUNITY. 



III. Observations on the Theories of Chemical 



Immunity. 



Immunity is due primarily to phagocytic activity, that indis- 

 pensable function of the body in the taking up and destruction of 

 bacteria or alien cells. Substances having a digestive effect on bac- 

 teria may also be found in the blood serum, particularly during the 

 condition of artificial immunity. As our knowledge of these active 

 substances of serum has increased it has become evident that they, 

 too, owe their origin to those phagocytic cells the function of which 

 is to protect the animal body. 



The cytolytic properties of serum have come to be regarded more 

 and more as simply a new manifestation of the activity of the pro- 

 tective cells. The tendency in the study of immunity has been to 

 harmonize these humoral manifestations with the functions of the 

 phagocytes, which, as Metchnikoff has shown, are both in origin 

 and function the digestive cells, fitted to form substances that digest 

 and destroy alien cells. 



But these substances in serum are themselves worthy of study 

 apart from any consideration of their origin. The study of this 

 "immunity of a chemical nature" has consisted hitherto in a com- 

 parison between the sera of normal animals and the sera of animals 

 that are in a condition of artificial immunity. In the sera of these 

 latter animals, particularly, the existence of specific cellulicidal 

 properties, frequently of great intensity, has been noted.* But 

 this cytolytic power is not the only characteristic of immune sera; 

 they have also the curious property of creating on injection an 

 intense cellulicidal property in the serum of a normal animal, as 

 Fraenkel and Sobernheim first pointed out in 1894. It is notable 

 that immune sera still retain this transferring property, even after 

 they have lost their own power to destroy cells by being heated to 

 55 degrees. In the example given by Fraenkel and Sobernheim, 

 cholera serum, whetker intact or heated to from 55 degrees to 60 

 degrees, when injected into a normal animal endows the latter's 

 serum with an intense bactericidal power. As another example it 

 may be noted that hemolytic serum from a rabbit "vaccinated" 



* We owe our conception of the specificity of bactericidal phenomena to 

 Pfeiffer's researches particularly. 



