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 
