ON IMMUNITY TO BACTERIA 339 



and dies of septicaemia. If dead vibrios are used the addition of 

 serum may bring about death, whilst an animal injected with the 

 organisms alone may recover with scarcely a symptom of intoxica- 

 tion. Similar phenomena, though less definite, often follow the 

 use of antistreptococcic serum for patients suffering from strepto- 

 coccic disease : the injection may cause a rapid rise of the 

 temperature and exacerbation of the symptoms. Other explana- 

 tions are possible, but it is highly probable that this is due to the 

 solution of some of the cocci and liberation of their endotoxin. 

 It is possible that in some cases this might be a source of danger, 

 and that this liberated toxin might be sufBcient to kill the patient ; 

 but there is no evidence that this actually occurs, and, as a rule, 

 the phenomena mentioned may be taken as proof that the serum 

 is a suitable one, and that its use should be continued. 



We must, however, be careful in arguing from the phenomena 

 occurring in hypervaccinated animals to those suffering from a 

 natural infection. There can be no analogy between the train of 

 events when a massive dose of pathogenic bacteria is suddenly 

 injected into the circulation or peritoneal cavity of an animal in 

 which there is already an abundance of bacteriolytic substances, 

 and those which follow a natural infection. Considering the 

 latter case, and excluding all considerations of phagocytic action, 

 we may trace the sequence of events in the case of a disease such 

 as typhoid fever somewhat as follows : In the first place, since 

 normal human serum is bactericidal to typhoid bacilli, we must 

 regard the normal resistance against the disease as being due, in 

 part at least, to the presence of immune body in the serum, 

 though we have no means of knowing whether its action is mainly 

 opsonic or mainly bacteriolytic when occurring in the plasma. 

 When infection occurs, the amount of this substance must 

 diminish, since it will combine with the typhoid bacilli to which 

 it gains access, and may, indeed, be sufBcient to kill them all, and 

 thus prevent the development of the disease. Where, however, 

 the organisms gain access in sufficient numbers, so that the amount 

 of immune body or of alexin is insufficient to prevent the further 

 growth of some of them, those that escape will continue to grow, 

 and the disease will gradually increase in severity. At the same 

 time, the dissolved products of the organisms which succumb will 

 reach the seats of antibody production and stimulate a fresh pro- 

 duction of immune body. W T e have seen reason to believe that 

 this production takes place mainly in the lymphoid tissue and 



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