8o INFECTION AND IMMUNITY 



the injection into susceptible animals of sterilised 

 cultures of the anthrax bacillus. More recently (1890) 

 Behring and Kitasato have shown that animals may 

 be made immune against the pathogenic action of 

 the bacillus of tetanus or the bacillus of diphtheria 

 by the injection of filtered, germ-free cultures of these 

 bacilli. 



In Pasteur's protective inoculations against hydro- 

 phobia, it is probable that the immunity which is de- 

 veloped after infection by the bite of a rabid animal 

 is due to the toxin of this disease present in the 

 emulsion of spinal cord which is used in these in- 

 oculations. 



We have also experimental evidence that animals 

 may acquire an artificial immunity against certain 

 poisonous substances of animal and vegetable origin. 



By inoculations with minute and gradually increas- 

 ing doses, animals may be made immune against rattle- 

 snake venom, and there is reason to believe that 

 persons who have been repeatedly stung by poison- 

 ous insects mosquitoes, bees acquire a considerable 

 degree of immunity from the distressing local effects 

 of their stings. 



Professor Ehrlich, of Berlin, in 1891, published the 

 results of some researches which have an important 

 bearing upon the explanation of acquired immunity, 

 and which show that susceptible animals may be 



