560 Chapter XVII 



As, in the majority of cases of acquired immunity, the blood 

 serum contains fixatives in considerable proportion, and as these 

 fixatives aid the action of the cytases in a remarkable fashion, we 

 can readily understand that the introduction of such a blood serum 

 into a normal animal, unprepared by any vaccination, may bring 

 about a great resistance against the corresponding pathogenic micro- 

 organisms. The fixatives, injected with the serum, fix themselves 

 with avidity upon the micro-organisms. These organisms may 

 become a more ready prey to the phagocytes and be destroyed 

 very rapidly. In particular cases, where the injection of microbial 

 cultures sets up a phagolysis, enough cytases are thrown out to 

 affect the microbes already sensibilised by the fixative. This is 

 followed by a refractory condition of the animal proportionate, in 

 general, to the amount of fixative serum that is injected. This 

 kind of acquired immunity, conferred by serums or certain other 

 body fluids rich in fixative substances, has often received the name 

 of passive immunity. This term is only justified in those rare cases 

 where the introduced serum itself contains a sufficient amount of 

 cytases to destroy all the micro-organisms. Most often it is the 

 normal animal which has to furnish this bacteriolytic ferment. 

 Now, as in phagolysis the quantity given off* is too small, it is to 

 the co-operation of the holders of cytases, that is to say, to the 

 phagocytes, that the animal must have recourse. The phagocytes, 

 being susceptible cells, their co-operation can only be counted upon 

 in cases where they exhibit a sufficient activity. When these 

 elements are weakened by narcotics or by any other cause, they 

 become incapable of intervening with efficacy and the animal falls 

 a victim to the pathogenic micro-organisms, in spite of the more than 

 sufficient amount of fixatives that was introduced. 



In natural or acquired immunity, it is the resistance of the animal 

 against the micro-organisms which plays the principal part. The 

 introduction of toxins ready prepared is only done under artificial 

 conditions, as in laboratory experiments. Hence we see that, under 

 natural conditions, it is against the penetration of the micro- 

 organisms that the animal must be protected. So soon as these 

 producers of poisons can no longer maintain themselves in the 

 immunised animal their toxic secretions do not come into play. 

 It is for this reason that animals vaccinated against pathogenic 

 micro-organisms do not suffer from intoxication, although they are 

 [585] by no means insusceptible to the microbial poisons. It is a fact 



