310 INFECTION AND IMMUNITY 



acter of the environment, chemical and physical, and the physical, 

 chemical, and physiologic characteristics of the germ involved. 



Having now considered the bacteria in their defensive and offensive 

 mechanism, let us turn to the mechanism of protection at the disposal 

 of the animal body. 



The internal defenses of the animal body have largely been eluci- 

 dated, as we have seen, through morphologic investigation of cellular 

 activities taking place in the animal body or under controlled condi- 

 tions in the test tube, and by visible reactions taking place in test tubes 

 between the fluids of normal or immunized animals and the bacteria 

 and their products, and, finally, by the more purely physiologic tests 

 of the protecting power and mechanism of action of animal fluids or 

 extracts when introduced into another animal of the same or different 

 species, along with the bacteria or their products. 



Such studies have, as is well known, afforded a vast amount of in- 

 formation. Through them the soluble secreted bacterial poisons have 

 been demonstrated and have been found to stimulate the production of 

 neutralizing bodies, the antitoxins; bacteria and their culture filtrates 

 have been shown to call forth bodies which are present in the serum of 

 animals treated with them, and which cause a precipitation of certain 

 bacterial constituents of the filtrate the precipitins; similar injections, 

 have been found to cause the production of serum bodies which have 

 the power of agglutinating the bacteria when brought into contact with 

 them the agglutinins; and other bodies are likewise produced which 

 are capable under proper conditions of killing the bacteria the bac- 

 tericidal substances or even of dissolving them as we have seen in 

 some instances the bacteriolytic substances. All of these activities, 

 present to a certain extent in normal serum but vastly increased in 

 immune sera may perhaps, as formerly claimed, be due to separate 

 serum constituents, but it is our own view at present that they repre- 

 sent different activities of the same specific type of antibody, namely, 

 a sensitizer in the sense of Bordet, by the agency of which the antigen 

 is rendered susceptible either to the action of the alexin or to 

 phagocytosis. Each antigen injected into the animal body would 

 thus give rise to a specific sensitizer, or, in the language of 

 Ehrlich, amboceptor. The union of the antigen with this am- 

 boceptor alters it so that, if a whole cell, it is subjected to ag- 

 glutination by electrolytes and other influences; if a dissolved 

 protein, it becomes precipitable and at the same time is rendered 

 amenable to the action of alexin and can be more easily taken 



