BACTERICIDAL OK ANTI-BACTERIAL IMMUNITY. 195 



its toxins, or against special toxic substances, affords protection against 

 infection or intoxication by entirely different agents. Thus animals 

 may be immunized against anthrax by inoculation with bacillus pyo- 

 cyaneus. 



It should be borne in mind, however much importance we may attach 

 to the formation and action of the antitoxic substances, that these are 

 not necessarily always present either in natural or acquired immuuitj^ to 

 bacteria or their toxins. Tolerance to bacterial toxins may be estab- 

 lished, as may tolerance to other kinds of poisons, without the interven- 

 tion of antitoxic or other chemical agents. 



The Complexity of the Processes Involved in Immunization. It thus 

 appears that while we know a great deal about the ability of the living 

 body to protect itself against the incursions of micro-organisms and the 

 ravages of their poisons; while a field is opened for the study of artificial 

 immunization which is of the highest promise, both for the advancement 

 of science and for practical benefit to the victims of infectious disease ; 

 while illuminating and far-reaching hypotheses are current which account 

 for many of the complex phenomena, we are yet very far from compre- 

 hending many of the details of the processes by which immunization is 

 secured. 



We do not know why the cells of certain animals or why different 

 kinds of cells in the same animal are more susceptible than others to the 

 presence of particular poisons ; why, for example, the rabbit is less sus- 

 ceptible than man to morphiu ; why strychnin should affect the nerves 

 while curare acts upon the muscles ; why the common fowl should be 

 extremely insusceptible to the tetanus toxin so powerful in many other 

 animals. We are even ignorant as yet in most cases of either the chemi- 

 cal or structural changes in cells by which the deleterious action of poisons 

 is effected. This is indeed not surprising when we reflect that the proc- 

 esses which are involved are of the most subtle and complex nature and 

 that our knowledge of cell metabolism even under normal conditions is 

 most crude and fragmentary, consisting largely in rather gross determi- 

 nations of end-products and leaving out of the account the numberless 

 molecular transformations and combinations through which the life proc- 

 esses of the cell are carried on. 



The living body cell is very nicely adapted to its normal environ- 

 ment ; the living bacterium is almost equally sensitive to the conditions 

 under which its metabolism takes place. Thus it is that when these 

 subtle organisms react upon each other, we are wholly unable with our 

 present knowledge to follow the steps by which the mor^ gross mani- 

 festations of disturbance which we call disease are reached. 



But there seems to be abundant ground for the belief that the pro- 

 tective agencies which are evoked in both natural and artificial immuni- 

 zation are simply those which the body makes use of in its normal metab- 

 olism, exaggerated and diverted to different ends, it is true, in the face 

 of emergencies and the establishment of new-cell environments, but giving 

 evidence of the birth of no new physiological capacities. 



