246 INFECTION AND IMMUNITY. 



kill bacteria in the test-tube, and after additional 

 facts were obtained by Nuttall, Buchner demon- 

 strated that it is not necessary to use the full blood 

 in order to obtain the bactericidal action, but that 

 serum alone has a similar effect. He spoke of the 

 antibacterial substances collectively as alexins 

 (substances which ward off), taking the reason- 

 able view that natural immunity to bacteria de- 

 pends on their presence in the body. The increased 

 bactericidal power of the serum which develops 

 during immunization or infection with certain 

 micro-organisms goes hand in hand with the in- 

 creased resistance of the individual against the 

 infection. The alexins have undergone a specific 

 increase; they are now immune alexins or, as we 

 say to-day, immune bacteriolysins, and it is sup- 

 posed that acquired immunity, in these instances, 

 depends on their new formation. 

 selective Alexins were very sensitive substances ; they dis- 



Action. , J , . ? - 



appeared spontaneously from serums in a few 

 days, were destroyed by a rather low degree of 

 heat (55 C.), by acids and alkalies, and were 

 active only in the presence of certain salts, espe- 

 cially sodium chlorid. A striking feature of 

 alexins, as distinguished from chemical bacteri- 

 cides, was their marked selective action on bac- 

 teria. The alexins of animal A might destroy one 

 micro-organism readily and affect another little or 

 none at all, whereas those of animal B might have 

 different selective characteristics. 



The Work which was instituted by Pfeiffer and de- 



veloped further by others led the way to a more 

 correct understanding of the nature of alexins. 

 Pfeiffer studied the bactericidal action of serums 



