IMMUNITY. 2 7 



a solution of abrin, of i to 10,000, caused a violent inflam- 

 mation. In this connection we may remark that there is 

 some evidence to show that persons who are repeatedly stung 

 by certain poisonous insects — mosquitoes, bees — acquire a 

 greater or less degree of immunity from the distressing local 

 effects of their stings. 



We have also experimental evidence that animals may 

 acquire a certain degree of immunity from the toxic action of 

 the venom of the rattlesnake. This was first demonstrated by 

 Sewall (1887), and has been recently confirmed by Calmette 

 (1894). 



The experimental evidence recorded justifies the conclusion 

 that in the diseases referred to acquired immunity depends, 

 chiefly at least, upon the presence of a peculiar proteid sub- 

 stance in the blood of the immune animal — antitoxin — 

 which neutralizes the toxic substance — toxin or toxalbumin 

 - — to which the morbid phenomena which characterize the 

 disease are due. 



But it would be premature to infer that in all infectious 

 diseases immunity depends upon the production of an antitoxin 

 in the blood of the immune animal. Indeed, we have experi- 

 mental evidence which shows that in certain cases the blood- 

 serum of immune animals has no antitoxic power, but acts 

 upon the germ itself, instead of upon its toxic products. 



It may be worth while to refer briefly, before closing, to 

 some examples of acquired immunity of a different order. We 

 refer to the tolerance of extremes of heat and cold which may 

 be established by habitual exposure, and more especially to the 

 tolerance to narcotics and irritant poisons, which is very 

 remarkable, and has never been explained in a satisfactory 

 manner. A recent writer (Samuel, 1892) has presented experi- 

 mental evidence which shows that the local inflammation which 

 results from the application of croton oil to the ear of a rab- 

 bit does not occur when a second application is made to the 

 same ear after recovery from the effects of the first. That a 

 tolerance may be acquired to comparatively large doses of 

 arsenic is well known, and the tolerance which the victims of 

 drug habits acquire to enormous doses of narcotics is a matter 



