THE PHENOMENA OF INFECTION 439 



as the pathogenic bacteria, but not being able to feed upon 

 the proteins of the body they die. This, however, is not 

 the sole, and probably not the most important, cause 

 of the failure of so many varieties of bacteria to do harm 

 to the higher animals. What has been said about the pro- 

 duction of ferments by the bacterial cell is equally true of the 

 body cell. In fact, it is true of every living cell. The body 

 cell has its specific ferments, and the bacterial cell being 

 protein substance is liable to be digested by the ferments 

 elaborated by the body cells. 



In the inability of the bacterial cell to grow in the animal 

 body either because it cannot feed upon the proteins of 

 the body, or because it is itself destroyed by the ferments 

 elaborated by the body cells lies the fundamental expla- 

 nation of all forms of bacterial immunity either natural 

 or acquired. Toxin immunity needs further explanation. 

 Certain bacteria, of which the diphtheria bacillus may be 

 taken as a type, elaborate soluble, extracellular substances 

 known as toxins. These are probably ferments or closely 

 allied bodies. They resemble ferments in the following 

 particulars: (1) They are destroyed by heat. (2) They 

 act in very dilute solution. (3) When repeatedly injected 

 into animals in non-fatal doses they cause the body cells 

 to elaborate antibodies which neutralize the toxin both 

 in vivo and in vitro. (4) In the development of their effects 

 a period of incubation is required. (5) It has been shown 

 by Abderhalden, by optical methods, that they have a 

 cleavage effect upon proteins. They split complex proteins 

 into simpler bodies. In other words, they have a proteo- 

 lytic action. (6) They are specific in two senses, (a) They 

 are specific according to the cell which produces them. 

 Diphtheria toxin is elaborated by the diphtheria bacillus 

 and by no other organism. The toxin of snake venom is a 

 specific product of the poisonous gland of the snake, and 

 this is further specific inasmuch as that produced by the 

 glands of one species is different from that elaborated in 

 another species. (6) They are specific in the antibody 

 elaborated in* the animal body after repeated injections of 



