INFECTION AND IMMUNITY 359 



When the agglutination of bacteria was discovered, 

 and observers everywhere were trying to account for the 

 peculiar phenomenon, Metchnikoff attributed it to the 

 action of one of these enzymes and looked upon it as a 

 preparation of phagocytosis. 



But the theory expanded most beautifully when it 

 became necessary to account for the phenomena of 

 cytolysis. Ehrlich's explains it as resulting from the 

 combination of complement, amboceptor, and cell. 

 Metchnikoff regards it as the result of the successive 

 action of the two enzymes. One, probably the macro- 

 cystase, prepares the cell by fixing or sensitizing it, the 

 other, the microcytase, then dissolves it. For this rea- 

 son Metchnikoff never uses the term amboceptor, but 

 speaks of that factor in cytolysis as the fixateur or sub- 

 stance sensibilisatrice. 



When Wright discovered certain substances in the 

 blood which he called opsonins, and which he believes 

 prepare the bacteria for phagocytosis, it seemed to 

 Metchnikoff but a new application of the fixateur. 



There are, therefore, two means by which the infect- 

 ing microparasites may be destroyed in naturally 

 immune animals the phagocytic cells and the germi- 

 cidal body humors and it makes comparatively little 

 difference by what theories we account for their 

 action. 



We must next endeavor to find out whether the same 

 conditions obtain in acquired immunity, but before at- 

 tempting this it may be well to pause to inquire how 

 immunity against infection may be acquired. 



It has already been pointed out that there are many 

 diseases from which one usually suffers but once. 

 Though a few notable exceptions occur, it is well known 

 that to have had chicken-pox, measles, scarlatina, mumps, 

 whooping-cough, yellow fever, typhoid fever, and small- 

 pox is to be immune against future attacks. The ex- 

 ceptions are of interest because they coincide with the 

 results of experimental investigation. 



