THEORIES AS TO ACQUIRED IMMUNITY 585 



that even though the complement should be taken up, the 

 zymotoxic group of the latter is hot sufficiently active towards 

 the bacterium to effect its death. In both cases it will appear 

 that an extracellular bactericidal action cannot be produced by 

 the particular immune-body in association with the complement 

 of the animal in question. There is no doubt that this question 

 of complements is one of high importance, and that both com- 

 bining affinity and toxic action of complements must be con- 

 sidered in each case. 



Theories as to Acquired Immunity. 



The advances made within recent years in our knowledge 

 regarding artificial immunity, and the methods by which it may 

 be produced, have demonstrated the insufficiency of various 

 theories which had been propounded. Only a short reference 

 need be made to these. The theory of exhaustion, with which 

 Pasteur's name is associated, supposed that in the body of the 

 living animal there are substances necessary for the existence of 

 a particular organism, which become used up during the sojourn 

 of that organism in the tissues ; this pabulum being exhausted, 

 the organisms die out. Such a supposition is, of course, quite 

 disproved by the facts of passive immunity. According to the 

 theory of retention, the bacteria within the body were considered 

 to produce substances which are inimical to their growth, so that 

 they die out, just as they do in a test-tube culture before the 

 medium is really exhausted. Such a theory only survives now 

 in the view that antitoxins are modified toxins, the evidence 

 against which has already been discussed (p. 567). There then 

 came the humoral theory and the theory of phagocytosis, but 

 neither of these is tenable in its pure form, and the distinction 

 between them need not be maintained. For, on the one hand, 

 any substance with specific property in the serum must be the 

 product of cellular activity, and, on the other hand, the facts 

 with regard to passive immunity go far beyond the ingestive and 

 digestive properties of phagocytes, though these cells may be in 

 part the source of important bodies in the serum. At the pre- 

 sent time interest centres around two theories, namely, Ehrlich's 

 side-chain theory and Metchnikoff's phagocytic theory as further 

 developed. These will now be discussed, and it may be noted 

 that the ground covered by each is not coextensive. For the 

 former deals chiefly with the production of anti -substances and 

 its biological significance, the latter deals with the defensive 

 properties of cells, either directly by their phagocytic activity 



