IMMUNITY AND SUSCEPTIBILITY. 69 



pends upon the antibactericidal action of their body- 

 juices must remain an open question. In some cases the 

 germicidal action of the blood seems to be unquestion- 

 able. Buchner has shown that the blood-serum of ani- 

 mals only possesses this germicidal power when freshly 

 drawn, and that exposure of the serum to sunlight, its 

 mixture with the serum from another species of animal, 

 its mixture with distilled water or with dissolved cor- 

 puscles, and heating it to 55 C., check the bactericidal 

 power. Buchner also points out that the bactericidal 

 and globulicidal actions of the blood are simultaneously 

 extinguished. 



Much discussion has arisen as to exactly what the pro- 

 tective substances are. Buchner has applied the term 

 alexin to the protective proteid substances found in the 

 blood of naturally immune animals. Hankin has given 

 us, together with an extension of Buchner' s idea, a con- 

 siderable nomenclature of somewhat questionable utility. 

 He divides the protective substances (alexins) into sozins, 

 which occur in the blood of animals with natural immu- 

 nity, and phylaxins, which occur in the blood of animals 

 with acquired immunity. Both sozins and phylaxins are 

 divisible into two groups thus : a sozin which acts de- 

 structively upon bacteria is called a myco-sozin; one 

 which neutralizes bacterial poisons, a toxo-sozin. A phy- 

 laxin which acts destructively upon bacteria is called a 

 myco-phylaxin ; one which neutralizes bacterial toxins, 

 a toxo-phylaxin. A glance will show that this classifi- 

 cation is based upon the somewhat doubtful existence 

 of alexins. 



5. THE THEORY OF ANTITOXINS. It is a well-known 

 fact that individuals can accustom themselves to the use 

 of certain poisons, as tobacco, opium, and arsenic, so as 

 to experience no inconvenience from what would be poi- 

 sonous doses for other individuals. This is purely a matter 

 of tolerance, but is of interest in connection with the 

 observations which are to follow. 



Ehrlich has shown that animals can tolerate gradually- 



