252 SUSCEPTIBILITY AND IMMUNITY. 



body of an immune animal, we must admit that the animal has ac- 

 quired a tolerance to the pathogenic action of these toxic substances, 

 for their presence no longer gives rise to any morbid phenomena. 

 And this being the case, we are not restricted to the explanation 

 that immunity depends upon a restraining influence exercised upon 

 the microbe when subsequently introduced. 



The Vital Resistance Theory. Another explanation offers itself, 

 viz., that immunity depends upon an acquired tolerance to the 

 toxic products of pathogenic bacteria. This is a view which the 

 writer has advocated in various published papers since 1881. In a 

 paper contributed to the American Journal of the Medical Sci- 

 ences in April, 1881, it is presented in the following language: 



"The view that I am endeavoring- to elucidate is that, during a non- 

 fatal attack of one of the specific diseases, the cellular elements implicated 

 which do not succumb to the destructive influence of the poison acquire a 

 tolerance to this poison which is transmissible to their progeny, and which 

 is the reason of the exemption which the individual enjoys from future 

 attacks of the same disease." l 



In my chapter on "Bacteria in Infectious Diseases," in "Bac- 

 teria," published in the spring of 1884, but placed in the hands of the 

 publishers in 1883, I say: 



" It may be that the true explanation of the immunity afforded by a mild 

 attack of an infectious germ disease is to be found in an acquired tolerance to 

 the action of a chemical poison produced by the microorganism, and conse- 

 quent ability to bring the resources of nature to bear to restrict invasion by 

 the parasite. " 



The "resources of nature" are referred to in the same chapter as 

 follows : 



' ' The hypothesis of Pasteur would account for the fact that one individual 

 suffers a severe attack and another a mild attack of an infectious disease, 

 after being subjected to the influence of the poison under identical circum- 

 stances, by the supposition that the pabulum required for the development 

 of this particular poison is more abundant in the body of one individual 

 than in the other. The explanation which seems to us more satisfactory is 

 that the vital resistance offered by the cellular elements in the bodies of 

 these two individuals was not the same for this poison. It is well known 

 that in conditions of lowered vitality resulting from starvation, profuse 

 discharges, or any other cause, the power to resist disease poisons is greatly 

 diminished, and, consequently, that the susceptibility of the same individual 

 differs at different times. 



"From our point of view, the blood, as it is found within the vessels of a 

 living animal, is not simply a culture fluid maintained at a fixed tempera- 

 ture, but under these circumstances is a tissue, the histological elements of 

 which present a certain vital resistance to pathogenic organisms which may 

 be introduced into the circulation. 



" If we add a small quantity of a culture fluid containing the bacteria of 

 putrefaction to the blood of an animal, withdrawn from the circulation into 

 a proper receptacle and maintained in a culture oven at blood heat, we will 

 find that these bacteria multiply abundantly, and evidence of putrefactive 



1 " "What is the Explanation of the Protection from Subsequent Attacks, result- 

 ing from an Attack of Certain Diseases, etc ? '' American Journal of the Medical 

 Sciences, April, 1881, p. 37. 



