82 BACTERIOLOGY 



ity in the production of which the cells of the organism 

 take no part. It is probably always artificial, and is ex- 

 emplified in the use of an immunizing dose of diphtheria 

 antitoxin or of tetanus antitoxin. The antitoxin is in- 

 troduced ready prepared into the blood of the organism, 

 whose cells are, therefore, under no necessity of pro- 

 ducing further antitoxin, and so take no active 

 part in the process. Such immunity is less lasting than 

 the active form because the antitoxin is soon eliminated, 

 while in the active form of immunity the cells go on 

 elaborating antitoxin and other antibodies for a consid- 

 erable tune after the recovery from the disease or in- 

 oculation whereby the immunizing process was brought 

 into action. 



In the study of active immunity numerous factors 

 must be taken into consideration, all of which have a 

 part in this complex phenomenon. Of these factors, 

 first place is probably occupied by the phagocytes, as 

 they are doubtless concerned in the resistance of all 

 forms of infection. Then there are the opsonins, with- 

 out which the phagocytes are helpless. The antibodies, 

 consisting of antitoxins, bactericides (alexins, etc.), and 

 bacteriolysins, and lastly, the agglutinins, of whose 

 part in the process we are yet more or less in the dark. 



Of these various substances and processes doubtless 

 in certain infections one plays a leading r61e, the others 

 being content with minor parts, while in other diseases 

 their relative importance may be reversed, depending 



