INFECTION AND IMMUNITY. 173 



thus engaged are called phagocytes, plays a most important part in the 

 establishment of immunity. On the other hand, certain ingredients of 

 the body fluids, formerly called alexins or "defensive proteids," acting 

 outside of cells, have been shown to possess marked germicidal powers. 

 While thus it might appear that two fairly distinct agencies are of im- 

 portance in enabling the body to resist the incursions of pathogenic 

 germs cellular or " phagocy tic, " and what maybe called the "humoral " 

 or chemical it is obvious that ultimately w r hatever destructive power 



In fluid froua the peritoneal cavity of an immunized guinea-pig a few hours after the introduction of the 

 hacilli of typhoid fever. Degenerative changes are seen in many of the bacteria within the cells. 



the body possesses toward micro-organisms must be due, directly or in- 

 directly, to cell activities. This phase of immunity has been character- 

 ized as anti-microbic, anti-bacterial, or bacteriolytic immunity, because it is 

 a mode of protection in which bacteria are destroyed or their growth in 

 the body is limited or prevented. 



Xot less important is another phase of immunity antitoxic immunity 

 by which the body protects itself against the toxic substances through 

 which, in many instances, the most serious manifestations of infection 

 occur. 



These two forms of immunity antitoxic and auti- bacterial or bac- 

 teriolytic will be considered in detail in a following section. 



Furthermore, the microbic toxins which are set free by the micro- 

 organisms, like other poisons may be eliminated together with the waste 

 products of body metabolism through the kidneys, skin, etc. 



ARTIFICIAL IMMUNIZATION. 



General Methods and Principles of Artificial Immunization. It was not 

 possible to gain a clear conception of the factors entering into the com- 

 plex problems of immunity until animal experimentation had revealed a 

 host of significant facts bearing directly upon questions which. for a long 

 time had seemed insoluble. It has been found as the result of experi- 

 mental researches, that artificial immunity can be secured by gradually 

 rendering the body tolerant to the presence of the infective or toxic 

 agencies without actually inciting the characteristic specific disease. 



I. In one class of procedures artificial immunity is secured directly or in- 

 directly through the action in the body of bacteria or other microbes or microbic 



