BACTERICIDAL OR ANTI- BACTERIAL IMMUNITY. 181 



this book does not permit us to consider the many and ingenious experiments by which 

 this view of antitoxic immunity has been sustained, nor is it practicable now to call 

 attention to many of the phenomena not yet accounted for or seemingly inconsistent 

 with the interpretations here set forth 



It is a working hypothesis which no doubt indicates but crudely the nature of the 

 subtle processes concerned, as must indeed be the case while our knowledge of the 

 phases of energy which sway and determine life are still very meagre, but it has 

 already inspired much fruitful research which is an important feature of working hy- 

 potheses in whatever tield, and whatever their ultimate fate. 



Bactericidal or Anti-bacterial Immunity (Bacteriolytic 

 Immunity). 



An extended series of studies on artificial immunization has shown, 

 as we have seen, that in relatively few instances, notably in diphtheria 

 and tetanus, is the protection secured by the formation of antitoxic sub- 

 stances. Nevertheless, when in some of the ways detailed above (p. 174) 

 an animal has been gradually adapted to cultures of pathogenic bacteria 

 either living or dead, or to their products, it has been found that the 

 body fluids contain protective substances. The protective action in these 

 instances in some cases has been shown to be associated with the induc- 

 tion of morphological changes in bacteria which indicate their damage 

 or destruction bacteriolysis. Active immunization in man wdth the 

 dead bodies of their respective bacteria has been widely practiced in ty- 

 phoid fever, Asiatic cholera, and plague, with apparently favorable re- 

 sults, while the use of the serum of immunized animals, in these and cer- 

 tain other diseases, has not been thus far very encouraging in conferring 

 passive immunity. 



While, therefore, the data at hand, in those artificial immunizations 

 which are not antitoxic, point to the germicidal and bacteriolytic action 

 of substances developed in the body as the important, if not the domi- 

 nant protective factors, there may be many other processes contributing 

 to the same end, which are as yet not clearly defined. Thus, there may 

 be increased phagocytosis ; vulnerable body cells may become less suscep- 

 tible, the growth of bacteria may be inhibited though they be not 

 destroyed, etc. But these possibilities cannot be discussed here. 



Furthermore, it should not be forgotten that the antitoxic action of 

 protective sera may often be associated with those agents which directly 

 damage the infecting organism. 



As regards the destruction of bacteria in the body, we have seen that 

 this may take place directly through the action of phagocytes or by the 

 action of the body fluids. But the details and exact nature of this de- 

 structive process have been extremely difficult of study, owing to their 

 complexity and the minuteness of the micro-organisms. Quite recently, 

 however, a series of remarkable studies in a related field have led to a 

 clearer conception of the ways in w r hich bacteria and many other alien 

 organic substances are destroyed in the body, under ordinary circum- 

 stances as well as under the special conditions which infection involves. 



