84 BACTERIOLOGY 



the toxin free before it can be neutralized by the anti- 

 toxin, but before the bacteriolysins can dissolve the 

 germs it is necessary for the bactericides or alexins to 

 destroy them. 



If these substances for a certain germ (for it should 

 be understood that they differ for different germs) 

 exist in sufficient quantities and the processes described 

 are sufficiently active, immunity to that germ must 

 exist. If these immunizing substances be not present, 

 the introduction of the germ into the body stimulates 

 their formation, and if recovery occurs, they remain in 

 the body in sufficient quantities to prevent any such 

 germ from obtaining a foothold in that territory for a 

 varying length of time. 



Whether natural immunity to toxin is due to the pres- 

 ence in the blood of antitoxin or to a lack of affinity 

 between the toxin and the body cells, or to both, we are 

 unable to say. It is a fact that in some instances anti- 

 toxin is found in the blood of healthy animals which 

 are immune to a certain toxin. In others no such anti- 

 toxin is present, and yet immunity exists, due perhaps 

 to a lack of affinity between the toxin and the cells of the 

 organism. The tortoise is unaffected by injections of 

 tetanus toxin, but if the blood of a tortoise which has 

 been injected with tetanus toxin be injected into a sus- 

 ceptible animal, death results, showing that the toxin 

 is not neutralized by the antitoxin or otherwise de- 

 stroyed. 



