28 BACTERIOLOGY 



disease, and which may be introduced into the blood of 

 other animals suffering from this same disease, where 

 they unite with the toxins of the disease, rendering them 

 harmless. At the same time they so affect the causative 

 germ as to check its growth, or else the germ exhausts 

 its soil or is attacked by the leukocytes, which are no 

 longer paralyzed by the toxins and aggressins; at any 

 rate, its activity ceases and recovery occurs. The most 

 important antitoxins are those of diphtheria and of 

 tetanus, of which more will be said later. 



Theories of Immunity. From the contents of the 

 foregoing pages it will be gathered that several theories 

 have been adduced in explanation of the phenomenon 

 of immunity. 



It will doubtless eventually be found to depend upon 

 the presence in the blood of these wonderful substances, 

 opsonins and antitoxins, either alone or in combination. 

 It is a fact already known that the injection of antitoxin 

 renders an individual immune to diphtheria for a greater 

 or less period of time, and raising the opsonic index by 

 vaccination against typhoid fever has been practised 

 with great success, rendering the vaccinated individual 

 immune to the disease in some instances for years. 

 It may be that in some diseases the immunity is the 

 result of phagocytosis, while in others it is due to the 

 antibodies, or, perhaps in some, a combination of both. 



