SUSCEPTIBILITY AND IMMUNITY. 267 
The experimental evidence detailed shows that in certain dis- 
eases acquired tmmunity 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 réle cannot be assigned to these agencies. 
As arule the antitoxins have no bactericidal action; but it has 
been shown by the experiments of Gamaléia, 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. 
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 
microérganism, 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 ofthe 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 
