SUSCEPTIBILITY AND IMMUNITY. 267 



The experimental evidence detailed shows that in certain dis- 

 eases acquired immunity depends upon the formation of anti- 

 toxins in the bodies of immune animals. As secondary fac- 

 tors it is probable that tolerance to the toxic products of pathogenic 

 bacteria and phagocytosis have considerable importance, but it is 

 evident that the principal role cannot be assigned to these agencies. 



As a rule the antitoxins have no bactericidal action; but it has 

 been shown by the experiments of Gamaleia, Pfeiffer, and others, 

 that in animals which have an acquired immunity against the spiril- 

 lum of Asiatic cholera and against spirillum Metschnikovi, there is a 

 decided increase in the bactericidal power of the blood serum, and 

 that immunity probably depends upon this fact. 9 



The researches of Metschnikoff upon hog cholera, of Issaef upon 

 pneumonia, and of Sanarelli upon typhoid fever indicate that the 

 immunity conferred upon susceptible animals by protective inocula- 

 tions is not due to an antitoxin but to a substance present in the 

 blood of immune individuals which acts directly upon the pathogenic 

 microorganism, as is the case in cholera-immune animals. The ani- 

 mals immunized are said to be quite as sensitive to the action of the 

 bacterial poisons as are those which have not received protective 

 inoculations. "Their serum does not protect against the toxin, but 

 against the microbe" (Roux) . 



According to Buchner (1894) the antitoxins are to be regarded not 

 as reactive products developed in the body of the immune animal, 

 but as modified, changed, and " entgiftete " products of the specific 

 bacterial cells. He insists that they do not neutralize the toxins by 

 direct contact, but only through the medium of the living organism. 

 This explanation scarcely appears tenable in view of the experimental 

 evidence, and the fact that the antitoxin of tetanus escapes in con- 

 siderable quantity with the milk of an immune goat without, ap- 

 parently, diminishing the immunity of the animal. In the immunity 

 against the toxic action of the vegetable toxalbumins ricin and 

 abrin as shown by Ehrlich's experiments, there are no " products of 

 bacterial cells " introduced with the pure toxalbumin from the castor 

 bean or the jequirity bean ; and we have sufficiently numerous ex- 

 periments to show that immunity, with the presence of antitoxins in 

 the blood, may be induced by precipitated and purified toxalbumins 

 from filtered cultures. Several of the experimenters, also, have re- 

 ported that the toxins from bacterial cultures are neutralized in vitro 

 by blood-serum from an immune animal, or by the precipitated anti- 

 toxin from such serum after contact for a certain number of hours. 

 If they are correct in the statement that a certain time is required 

 after the antitoxin has been brought in contact with the toxin, in 

 order that the latter may be neutralized, as shown by injection of the 



