SUSCEPTIBILITY AND IMMUNITY. 267 



ments showed that mice inoculated with virulent anthrax cultures 

 did not succumb to anthrax septicaemia if they received at the same 

 time a subcutaneous injection of a small quantity of the blood of an 

 immune animal. So small a dose as one drop of frog's blood or one- 

 half drop of dog's blood proved to be sufficient to protect a mouse 

 from the fatal effect of an anthrax inoculation. And the protective 

 inoculation was effective when made as long as seventy-two hours 

 before or five hours after infection with an anthrax culture. Fur- 

 ther, it was found that mice which had survived anthrax infection as 

 a result of this treatment were immune at a later date (after several 

 weeks) when inoculated with a virulent culture of the anthrax 

 bacillus. 



Behring and Kitasato have obtained similar results in their ex- 

 periments upon tetanus and diphtheria, and have shown that the 

 blood of an immune animal, added to virulent cultures before in- 

 oculation into susceptible animals, neutralizes the pathogenic power 

 of these cultures. . 



They have shown by experiment that the blood of a rabbit which 

 has an acquired immunity against tetanus, mixed with the virulent 

 filtrate from a culture of the tetanus bacillus, neutralizes its toxic 

 power. One cubic centimetre of this filtrate was mixed with five 

 cubic centimetres of serum from the blood of an immune rabbit and 

 allowed to stand for twenty-four hours ; 0. 2 cubic centimetre of this 

 injected into a mouse was without effect, while 0.0001 cubic centi- 

 metre of the filtrate without such admixture was infallibly fatal to 

 mice. The mice inoculated with this mixture remained immune for 

 forty to fifty days, after which they gradually lost their immunity. 

 The blood or serum from an immune rabbit, when preserved in a 

 dark, cool place, retained its power of neutralizing the tetanus tox- 

 albumin for about a week, after which time it gradually lost this 

 power. The blood of chickens, which have a natural immunity 

 against tetanus, was found not to have a similar power. Behring 

 and Kitasato have also shown that the serum of a diphtheria-immune 

 rabbit destroys the potent toxalbumin in diphtheria cultures. It 

 does not, however, possess any germicidal power against the diph- 

 theria bacillus. 



Ogata, in 1891, reported that he had succeeded in isolating from the 

 blood of dogs and of chickens a substance to which he ascribes the nat- 

 ural immunity of these animals from certain infectious diseases, and 

 the power of their blood to protect susceptible animals from the same 

 diseases. This substance is soluble in water and in glycerin, but in- 

 soluble in alcohol or ether, by which it is precipitated without being 

 destroyed. Its activity is neutralized by acids, but not by weak 

 alkaline solutions. Ogata supposes the substance isolated by him to 



