124 BACTERIOLOGY. 



neutralize in some way the protective substances (alex- 

 ines). The virulence of bacteria would, therefore, de- 

 pend partly upon their ability to produce these lysines, 

 which act perhaps as the ferments upon the alexines, or 

 perhaps combine with them. That bacteria under certain 

 conditions form specific poisons, and under others, even 

 when they grow luxuriantly, do not, is clearly shown by 

 our experiments on the production of diphtheria toxin. 

 Here, as previously stated, it was found that when 

 the bouillon was either a little too alkaline or too acid, 

 though the bacilli grew rapidly, they did not produce 

 specific toxins. By growing the bacilli for a time in 

 such bouillon they eventually became able to develop 

 toxin in a soil in which they previously failed to do so. 

 Similar cultivation in the body may be assumed to 

 increase their ability to produce specific poison after a 

 while under what would at first be adverse conditions. 



With regard to the increase and decrease of general, 

 and perhaps also of specific immunity, we have reason 

 to believe that as the protective substances are produced 

 by the living cells, anything which lowers the general 

 vitality must lessen the vitality of the cells, and thus 

 their ability to produce protective substances in the 

 amount possible in a normal condition. The attraction 

 of leucocytes to any point by some new infection might 

 increase the germicidal action of the tissues, and so in- 

 fluence the first infection. 



Specific Immunity. The following theories have been 

 advanced concerning the nature of specific immunity: 

 The theory that a second infection is impossible because 

 the first used up substances which were necessary to the 

 growth of the bacteria is untenable for many reasons. 

 Thus it can be demonstrated that the injection of a 



