490 IMMUNITY 



As has been shown above, antibacterial sera require for their 

 complete action a sufficiency of complements, and as these 

 diminish in amount when a serum is kept, the unsatisfactory 

 results with this class of sera may be due to a deficiency of 

 complement. Or it may be as Ehrlich has suggested, that the 

 complement naturally existing in human serum does not suit 

 the immune-body in the anti-serum that is, is not taken up 

 through the medium of the latter and brought into combination 

 with the bacterium. And there is still the further possibility 

 that even though the complement should be taken up, the 

 zymotoxic group of the latter is not 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 importance, and that both combining 

 affinity and toxic action of complements must be considered 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 oj 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. 470). 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 



