584 IMMUNITY 



cases of slight bite, however, benefit may accrue from the use of 

 the anti-serum. . 



As has been shown above, antibacterial sera require for their 

 bactericidal action a sufficiency of complement, and as this 

 diminishes 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 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 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 high importance, and that both combining 

 affinity and toxic action of complements must be considered in 

 each case. 



In such diseases as cerebro-spinal fever and pneumonia the 

 opsonic mechanism of the infected individual may play a part in 

 successful resistance. The favourable effects following treatment 

 with anti-sera may thus in some cases depend on an augmentation 

 of the opsonic powers of the body. 



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 



