144 CHEMISTRY OF IMMUNITY AGAINST BACTERIA 



distinct from the others and may therefore be considered by 

 itself. 



BACTERICIDAL SERUM 



The bactericidal property of serum may be shown by its 

 destruction of the life manifestations of bacteria without marked 

 alteration in their structure, or it may be accompanied by dis- 

 solution of the bacterial cell (bacteriolysis). How much of the 

 bacteriolytic process is performed by the serum itself, or how 

 much by the autolytic enzymes of the bacterial cell, is unknown, 

 but the latter is probably an important factor. The bactericidal 

 property of immune serum has been shown to be quite inde- 

 pendent of the antitoxic properties and also to have quite a dif- 

 ferent mechanism. This last is shown in the following manner : 



If we heat bactericidal serum made by immunizing an animal 

 against bacteria, say the cholera vibrio, at 55 for fifteen minutes, 

 it will be found to have lost its power of destroying these organ- 

 isms. Normal serum of non -immunized animals is equally with- 

 out effect upon the vibrios. If, however, we add to the inactive 

 heated serum an equal quantity of inactive normal serum, the 

 mixture will be found to be as actively bactericidal as the orig- 

 inal unheated immune serum. This phenomenon is interpreted 

 to mean that, by immunization, some new substance has been 

 developed which, although by itself incapable of destroying 

 bacteria, is able, when united with some substance present in 

 normal serum, to destroy bacteria readily. The substance pres- 

 ent in normal serum is also incapable of affecting bacteria by 

 itself, but needs the presence of the substance developed by 

 immunizing to render it bactericidal. Hence the bactericidal 

 property in this case depends on two substances acting together : 

 one, developed during immunization and therefore called the 

 immune body, is specific for the variety of bacteria used in immu- 

 nization, and is not destroyed by heating at 55. The other, 

 present in normal serum, is not increased during immunization, 

 is not (altogether) specific in character, and is destroyed by 

 heating at 55 ; as its action is complementary to that of the 

 specific immune body, it is called the complement. 



It is believed that the action of these substances is as follows : 

 The immune body is, like antitoxin, a cell receptor which unites 



by bacteria of marked selective action for specific cells of the host ; e. g., leucp- 

 lysins, endotheliolysins, hemolysins, etc. ; and also the peculiar manner in 

 which bacteria often attack only certain tissues, e. g., multiple septic arthritis. 

 The fact that bacteria are said to develop enzymes with specific effects accord- 

 ing to the media upon which they grow is in support of this hypothesis. 



