10 INFECTION AND IMMUNITY. 



reactions which may occur when bacteria or their 

 products are placed in contact with tissue cells 

 or fluids, either in the living animal or in test- 

 glass experiments. The chief of these are the de- 

 Bacterici- termination of the ability of the serum of an ani- 

 mal to kill bacteria or to neutralize bacterial 

 toxins, l^hese important investigations were in- 

 augurated by the findings of Fodor, Xuttall, Xis- 

 sen, v. Behring and Buchner, which showed 

 that fresh defibrinated blood, and the blood 

 serum of various animals, are able to kill bac- 

 teria in the reagent glass. In contrast to the 

 action of ordinary antiseptics, this power is 

 often selective, killing one variety of bacterium 

 and leaving another unharmed. This was of enor- 

 mous importance, as it seemed to identify the 

 factor on which natural antibacterial immunity 

 depends. Then followed the discovery of Nissen 

 and v. Behring (Vibrio metchnikovi) , and of 

 Bouchard (B. pyocyaneus), that if an animal is 

 systematically injected, i. e., immunized, with a 

 micro-organism, the power of its serum to kill 

 the bacterium used in the immunization is greatly 

 increased ; from which it would seem that acquired 

 immunity depends on the increase of powers which 

 are normally present to a certain degree. These 

 observations have to do with the bactericidal power 

 of serum. 



Toxins and Further progress was made through the discov- 

 eries that the tetanus bacillus (Brieger a/id Fran- 

 kel) and the diphtheria bacillus (Roux and Yer- 

 sin) secrete each a powerful, specific, soluble toxin, 

 which may be separated from the bacteria by fil- 

 tration. Immunization with these bacterium-free 

 toxins was undertaken (Behring and Kitasato, 



