LIFE OF BLIE METCHNIKOFF 177 



mechanism explains the specificity of the serums of 

 vaccinated animals. 



The quantity of specific fixators in the humors 

 depends on the surplus production of that ferment by 

 the phagocytes and is not always the same. That is 

 why different serums are preventive in different 

 degrees. They are inactive if the phagocytes have 

 not produced enough fixators to pass any out into 

 the humors. For a serum is only preventive when it 

 brings into the new organism into which it is in- 

 jected a sufficient quantity of fixators ready to 

 sensibilise the morbid agents afterwards introduced 

 into the organism. 



The over-production of antibodies — fixators or anti- 

 toxins — corresponds up to a certain point with the 

 frequency and quantity of vaccinal injections ; that 

 is why serums are usually preventive in artificial 

 immunity and very rarely so in natural immunity. 

 Through successive inoculations, the cells become 

 accustomed to digesting the microbes, or figured 

 elements, and manufacture, in consequence of that 

 digestion, growing quantities of fixators. 



In natural conditions, on the other hand, morbid 

 agents do not usually penetrate into the organism in 

 massive or repeated doses ; therefore digestion under 

 natural conditions results in a less abundant produc- 

 tion of fixators which can be contained in the interior 

 of the phagocytes without leaking into the humors 

 in sufl&cient quantities to render the latter preventive. 



It might be thought that immunity against patho- 

 genic microbes is accompanied by immunity against 

 their toxins. In reality that is not always the case, 

 and very often the organism, now made refractory 

 to certain microbes, remains sensitive to their toxic 



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