164 PRINCIPLES OF VETERINARY SURGERY 



soluble ferments, the cytases and fixatives, Metchnikoff ex- 

 plains the various types of immunity as follows: 



In natural immunity the microphages stand out as the 

 defensive cells par excellence against micro-organisms. 

 The macrophages may act in some degree, but there is no 

 help on the part of the body fluids. The phagocytes seize 

 and ingest the living micro-organisms which they dissolve 

 completely by means of their intracellular ferments, the 

 cytases. The agglutinative power of the normal fluids 

 plays no important part in natural immunity. 



In acquired immunity, there exist as a fairly general 

 rule, other substances whose role in the defensive action 

 offered by the animal against micro-organisms is very im- 

 portant. These are the fixatives, which permeate the bac- 

 teria and render them more susceptible to the bactericidal 

 action of the microcytase. These fixatives, however, al- 

 though found in the serum, are undoubtedly of cellular 

 origin, probably the phagocytes. Having absorbed the 

 micro-organisms, they set to work to elaborate large quan- 

 tities of fixatives, which they secrete and pass into the blood 

 plasma and other fluids. This excretion, however, is not 

 an indispensable act for the functioning of the fixatives. In 

 the cases of acquired immunity where no fixatives are 

 found in the body fluids, they remain lodged in the phago- 

 cytes, where they act in conjunction with the cytases. 



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

 blood-serum contains fixatives in considerable proportion 

 which aid the action of the cytases, we can understand that 

 the injection of such a serum into other animals would in- 

 crease their resistance also. The fixatives injected with the 

 serum, fix themselves with avidity upon the micro-organ- 

 isms and make them a more ready prey to the phagocytes. 

 Thus Metchnikoff explains passive immunity. 



Immunity is much more readily acquired against micro- 



