THEORIES OF IMMUNITY. 



The following summary of this part of the 

 subject is exceedingly good (Ritchie, I. c, p. 

 261) : It is to be observed that the methods by 

 which bacteria are dealt with in the body are 

 similar to those which obtain when many kinds 

 of foreign cells gain an entrance into the latter. 

 The development of artificial immunity against 

 such bacteria depends on the latter being intro- 

 duced either in a form not strong enough to 

 cause death, or, if virulent, not in sufficient 

 numbers to cause death. In either case, the 

 affected animal probably resists infection be- 

 cause it can develop in its body, or already 

 possesses a substance — immune body — which 

 attaches itself to the bacterial protoplasm, and in 

 virtue of this attachment permits another body, 

 the complement, which exists normally in the 

 animal's body, to act on the bacteria, with a 

 fatal result to the latter. In the case of a further 

 infection with bacteria, such as might occur 

 naturally, or as occurs during the process of 

 immunization, then no illness may result, but a 

 fresh formation of immune body may occur. 

 Whether a fresh formation of complement may 

 occur to any extent is a question for further 



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