STUDIES ON THE SERUM OF VACCINATED ANIMALS 79 



II. The Mechanism of the Immunity Conferred by Preventive 

 Serum and the Evidence for Considering this Immunity as a 

 Purely Chemical Phenomenon. 



There are certain conclusions to be drawn from the facts that we 

 have offered as to the nature of the immunity conferred by serum. 



Cholera serum, as we know, is not antitoxic; animals vaccinated 

 against cholera have no real immunity against cholera toxin. The 

 serum gives an immunity only against the organism itself. It 

 contains, when fresh, two substances: a bactericidal substance and 

 a preventive substance. Serum that has been kept for some time 

 or heated to 55 degrees, no longer contains the bactericidal sub- 

 stance. As Fraenkel and Sobernheim were the first to show, such 

 a serum, although deprived of its bactericidal substance, still 

 retains its immunizing properties. 



It is easy to understand why the presence of this bactericidal 

 substance is not indispensable for the preventive action of the serum. 

 It is due to the fact that the bactericidal substance is not a peculiar 

 property of this serum but exists in any normal serum, so that a 

 normal animal given an injection of cholera serum is protected 

 because it already possesses the bactericidal substance. 



The substance that the normal animal does not have is the pre- 

 ventive substance, and it is, therefore, this latter substance that it 

 is important to furnish. One may ask how this substance when 

 introduced into the tissues produces immunity. It is certain that 

 leucocytes, in common with other sensitive living cells, react to the 

 presence of the preventive serum. Stimulated by this serum they 

 give evidence of positive chemiotaxis. We know, moreover, that 

 in animals immunized by serum, phagocytosis occurs with remark- 

 able intensity. It is difficult, then, in view of these facts, not to 

 believe in the idea advanced by several observers and notably by 

 MetchnikofT, Roux, and Sanarelli that serum acts on cells as a 

 "stimulin" that excites phagocytosis. 



But there is another phenomenon in passive immunity which is 

 perhaps even more important. The injection of preventive cholera 

 serum causes the appearance of a very distinct bactericidal property 

 against the cholera vibrio, for which the serum injected is specific. 

 We know already that the preventive substance, incapable in itself 



