554 Chapter XVII 



numbers to the menaced spot. This apparent contradiction is easily 

 explained by the fact that it is in the serum only that the vibrios en- 

 counter the microcytase, which has escaped from the microphages at 

 the time of the formation of the clot and the separation of the serum. 

 [578] Alongside those cases in which the serum of susceptible animals 

 is found to be very bactericidal, examples are not wanting where 

 the blood and the serum of refractory animals are entirely without 

 this power. For instance, the pigeon is refractory to Pfeiffer's 

 influenza bacillus, but the blood of the pigeon forms the best culture 

 medium for this micro-organism. The dog is refractory to the 

 anthrax bacillus, against which the blood serum of the same animal 

 is not at all bactericidal. The cause of this absence of parallelism 

 between immunity and the bactericidal power of the serums must be 

 sought in the difficulty with which the cytases escape from the leuco- 

 cytes, and also in the modifications which they may undergo, once they 

 are distributed in the fluids. 



In cases of natural immunity, the cytases rid the animal of the 

 micro-organisms without the slightest observable co-operation on the 

 part of other soluble ferments. It is impossible to settle definitely 

 even the question whether, in animals which enjoy this innate im- 

 munity, there exists, alongside the microcytase, any ferments which 

 come to its aid. The conditions are quite otherwise in a very large 

 number of cases of acquired immunity. Here it is found, as a fairly 

 general rule, that in addition to the microcytases there exist other 

 substances whose role in the defensive action offered by the animal 

 against micro-organisms is very important. These substances are 

 fixatives which co-operate in a remarkable fashion with the bacteri- 

 cidal action of the cytases ; but whilst these latter injure the 

 bacterial cell directly, the fixatives do not interfere with its life. 

 The bacteria, permeated by fixatives, may even continue to reproduce 

 themselves and, under certain conditions, to invade the animal. The 

 fixatives, then, are not bactericidal, but by fixing themselves upon 

 the micro-organisms they render them much more susceptible to 

 the bactericidal action of the microcytases. These latter are further 

 distinguished, in several other respects, from the cytases. The 

 fixatives must also be classed with the group of soluble ferments, 

 but they resist much higher temperatures than those which destroy 

 the cytases. Whilst the latter are quite destroyed at 55° C, the 

 fixatives, to be completely altered, must be heated to beyond 60° C. 

 and even 65° C. On the other hand, the fixatives are distinguished 



