IMMUNITY 19 



the bactericidal power of normal serum, like immune 

 serum, must depend upon the presence of amboceptor 

 and complement. But normal serum often exerts 

 lytic powers upon several species of bacteria, or, in 

 the hemolytic tests, upon the red blood cells of several 

 species of animals. It is supposed that this multi- 

 plicity of action is due to the presence in the normal 

 serum of a variety of different amboceptors. The 

 immunity acquired by an animal as a result of treat- 

 ment with any of the various antigens is specific — 

 for instance, an animal immunised against anthrax 

 possesses marked bactericidal powers against the 

 anthrax bacillus only. 



The essential fact to remember is that the ambo- 

 ceptor alone enters into direct relation w^ith the sub- 

 stance used for immunisation, and the specificity of 

 immune sera therefore depends entirely upon the 

 increase of amboceptor or immune body, and there is 

 no corresponding increase of the complement ; so the 

 chief difference in a normal and immune serum, 

 therefore, is the enormous increase of the specific 

 amboceptors in the latter. 



It might seem, from a consideration of the fore- 

 going facts, that the explanation of immunity was 

 very simple, but other ascertained facts render the 

 question more complicated. Thus, taking the case of 

 anthrax bacilli, we find that the blood of the rabbit, 

 a highly susceptible animal, acts destructively upon 

 the organisms in a test-tube ; within the body it 

 evidently does not do so. On the other hand, the 



