IMMU]SriTY-GIVING SUBSTANCES. 149 



first place, if acquired immunity is of this nature, we ai-e 

 dealing with an acquired tolerance of a poison, which 

 tolerance is conferred by administering a single dose, or at 

 most a very limited number of doses. Further, this ac- 

 quired tolerance, thus easily obtained, is very permanent, 

 lasting for months, or even years. Now, though acquired 

 tolerance of alkaloids is constantly observed, it is but 

 limited in degree, and only obtained as the result of a 

 long-continued succession of doses.' Secondly, since ac- 

 quired tolerance of this hypothetical poison results in the 

 microbe being no longer capable of living in the body, 

 this theory implies that the poison in question is one that 

 is produced by the microbe in order to live there. In 

 other words, that it is a poison capable of lowering the 

 bacteria-killing power possessed by every living animal 

 body.' 



" Of course, it is conceivable that a ptomaine might be 

 concerned in doing this, but, so far as I know, no parallel 

 to such action can be found among bodies of an alkaloidal 

 nature. 



"When, however, we turn to what is known of poison- 

 ous proteids, we at once find that they have properties 

 analogous to those of the hypothetical immunity-giving 

 poison. 



" First, as regards the question of tolerance : Two poisons 

 are known, which, in the nature of the tolerance they pro- 

 duce, resemble the hypothetical poison in question. Both 

 of them are albumoses. The first is the ordinary hemi- 

 albumose of proteid digestion. It is known that the injec- 

 tion of a single minute dose confers immunity against a 



I Carbone claims to have obtained immunity in rabbits against the action 

 of the proteus vulgaris by means of not more than two previous injections 

 of small quantities of neurin obtained from cultures of the proteus. He 

 still further states that immunity against the same germ is obtained by 

 musoarin, which produces physiological effects practically identical with 

 those of this neurin. 



' With this statement we must take issue. The experiments already 

 given in which immunity is induced in a susceptible animal by the injection 

 ot the serum of the blood of an animal naturally immune show that the 

 immunity-giving substance is not necessarily of bacterial origin, and cer- 

 tainly that it is not necessarily a product of the germ against which the 

 immunity is secured. 



