SUSCEPTIBILITY AND IMMUNITY. 273 



animal, which has the power of destroying the toxic products of bac- 

 terial growth. 



A mycophylaxin is a defensive proteid produced in the body of 

 an animal which has an acquired immunity for a given infectious 

 disease, which has the power of destroying the pathogenic bacteria 

 to which the disease is due. 



A toxophylaxin is a defensive proteid produced in the body of 

 an animal which has an acquired immunity for a given infectious 

 disease, which has the power of destroying the toxic products of the 

 pathogenic bacteria to which the disease is due. 



Buchner had previously proposed the name " alexines " for these 

 defensive proteids. 



The importance of the experimental evidence above referred to in 

 explaining the phenomena of natural and acquired immunity is ap- 

 parent. The facts stated also suggest a rational explanation of re- 

 covery from an attack of an acute infectious disease. But the idea 

 that during such an attack an antidote to the disease poison is de- 

 veloped in the tissues is yet so novel, and the experimental evidence 

 in support of this view is of such recent date, that it would be pre- 

 mature to accept this explanation as applying to immunity in gene- 

 ral. It seems difficult to believe that an individual who has passed 

 through attacks of measles, mumps, whooping cough, scarlet fever, 

 small-pox, etc. , has in his blood or tissues a store of the antitoxine of 

 each of these diseases, formed during the attack and retained during 

 the remainder of his life, or continuously produced so long as the 

 immunity lasts. Moreover, in those diseases to which the experi- 

 mental evidence above recorded relates diphtheria, tetanus, pneu- 

 monia as they occur in man, no lasting immunity has been shown 

 to result from a single attack, and in this regard they do not come 

 into the same class with the eruptive fevers and other diseases in 

 which a single attack usually protects during the lifetime of the in- 

 dividual. 



In those instances in which acquired immunity has been shown 

 to be due to the production in the body of the immune animal of an 

 antitoxin, it is still uncertain whether there is a continuous produc- 

 tion of the protective proteid, or whether that formed during the 

 attack remains in the body during the subsequent immunity. The 

 latter supposition appears at first thought improbable ; but when we 

 remember that the protective proteids which have been isolated by 

 Hankin from the blood and spleen of rats, and by Tizzoni and Cat- 

 tani from the blood of animals made immune against tetanus, do 

 not dialyze, it does not seem impossible that these substances might 

 be retained indefinitely within the blood-vessels. On the other hand, 

 the passage of the tetanus antitoxin into the mother's milk, as 

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