SUSCEPTIBILITY AND IMMUNITY. 231 



experiments made by them relating to acquired immunity from the 

 pathogenic action of the bacillus of tetanus. The blood of animals 

 which had been made immune was injected into a susceptible animal, 

 and at the end of twenty-four hours it was inoculated with a virulent 

 culture. The result was negative, while control experiments made 

 with the same culture gave a uniformly fatal result. So small a 

 quantity as 0.2 cubic centimetre of blood from an immune rabbit, in- 

 jected into the cavity of the abdomen of a mouse, was sufficient to 

 protect it from the fatal effects of a virulent culture of the tetanus 

 bacillus injected twenty-four hours later. Further, these bacteriolo- 

 gists have shown that the toxic substances present in a- filtered cul- 

 ture of the tetanus bacillus are neutralized by admixture with the 

 blood of an immune rabbit. A culture ten days old was sterilized 

 by filtration ; 0.0001 cubic centimetre of the filtrate was found to kill 

 a mouse with certainty in less than two days. Of this filtered cul- 

 ture one cubic centimetre was added to five cubic centimetres of blood 

 serum from an immune rabbit. At the end of twenty-four hours 

 four mice received each 0. 2 cubic centimetre of the mixture, contain- 

 ing more than three hundred times the fatal dose of the filtered cul- 

 ture. All of these mice survived the injection and proved subse- 

 quently to be immune for virulent tetanus bacilli, while four control 

 mice, each of which was inoculated with 0.0001 cubic centimetre of 

 the same filtrate unmixed with blood, perished within thirty-six 

 hours. The blood of rabbits not immune was without effect in neu- 

 tralizing the toxic substances in a filtered culture of the tetanus ba- 

 cillus, as was also the blood of children, calves, sheep, and horses. 



The same bacteriologists have obtained similar results by mixing 

 the blood of an animal which had an acquired immunity against the 

 poison of the diphtheria bacillus with filtered cultures of this bacillus. 

 The toxic substances present are neutralized by such admixture, but, 

 according to Behring, the bacilli themselves are not destroyed by the 

 blood of an immune animal. 



It has also been shown by experiment that naturally Immune ani- 

 mals may be infected by the addition of certain substances to cultures 

 of pathogenic bacteria. Thus Arloing was able to induce symptomatic 

 anthrax in animals naturally immune by mixing with his cultures 

 various chemical substances, such as carbolic acid, pyrogallic acid, 

 and especially lactic acid (twenty per cent). Leo has shown that 

 white mice, which are not subject to the pathogenic action of the 

 glanders bacillus, may be rendered susceptible by feeding them for 

 some time upon phloridzin, which gives rise to an artificial diabetes 

 and causes the tissues to be impregnated with sugar. 



Before discussing the rationale of acquired immunity a state- 

 ment of certain established facts will ba desirable. 



