274 SUSCEPTIBILITY AND IMMUNITY. 



shown by Ehrlich's experiments upon mice, indicates a continuous 

 supply, otherwise the immunity of the mother would soon be lost. 



The writer has obtained (May, 1892) experimental evidence that 

 the blood of vaccinated, and consequently immune, calves contains 

 something which neutralizes the specific virulence of vaccine virus, 

 both bovine and humanized. Four drops of blood serum from a calf 

 which had been vaccinated two weeks previously, mixed with one 

 drop of liquid lymph recently collected in a capillary tube, after con- 

 tact for one hour was used to vaccinate a calf ; the same animal was 

 also vaccinated with lymph, preserved on three quills, which was 

 mixed with four drops of serum from the immune calf and left for 

 one hour. The result of these vaccinations was entirely negative, 

 while vaccinations upon the same calf made with virus from the 

 same source, and mixed with the same amount of blood serum from 

 a non-immune calf, gave a completely successful and typical result, 



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. 



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). 



