THEORIES AS TO ACQUIRED IMMUNITY. 489 



recognised. Behring showed that in cases treated on the first and 

 second days of the disease the mortality was only 7.3 per cent, 

 and this has been generally confirmed, whilst after the fifth day it 

 was of little service to apply the treatment. In order to obtain 

 such results it cannot be too strongly insisted on that attention 

 should be given to the dosage. When bad results are obtained 

 it may be strongly suspected that this precaution has not been 

 observed. In the treatment of acute tetanus by the antitoxin 

 the improvement in results has not been marked, but some 

 chronic cases have been benefited. In the case of Yersin's anti- 

 plague serum, though benefit has appeared to follow its use, 

 experience with its effects has been too limited to enable a 

 judgment to be formed. The same may be said to be true of 

 the antistreptococcic and antipneumonic sera, and also of anti- 

 venin, though in the case of the first mentioned numerous cases 

 of apparently successful result have been recorded. 



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. There is no doubt, however, 

 that this question of comple'ments is one of importance, and will, 

 in all probability, be cleared up in the further development of 

 research on this subject. 



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 



