28 THEORIES OF IMMUNITY 



follows that in mixtures of toxins with their antitoxins or in 

 general, of antigens with their antibodies, there are not 

 produced chemical combinations in the ordinary sense but 

 certain states of equilibrium which are variable for each 

 concentration or proportion of one product in the other; 

 in other words, if in certain proportions, two products can 

 exactly neutralize each other, either of them can fix its "anti" 

 en surcharge. 



We will see further how this theory which results from 

 facts established by very exact experiments is important in 

 explaining the biologic action of antigens. For the moment, 

 it is necessary to remember that antigens may form with 

 their antibodies compounds which are soluble (in the cases of 

 diphtheria toxin, tetanus toxin, or venins) or compounds 

 w"hich are insoluble (in the cases of ricin or abrin). We may 

 also say in this last case that the antibody precipitates its 

 antigen and all precipitating antibodies are called "pre- 

 cipitins." 



The discovery of antidiphtheritic serum gave birth to the 

 hope that it would be possible to prepare antibodies specific 

 for every infectious disease of which the bacteria were known. 

 Thus after the discovery of anthrax vaccine, it was hoped 

 that man and the domestic animals could be vaccinated 

 against all the contagions. It has been established that not 

 only all the microbes and their secretions are "antigens" 

 but that many other substances have a similar property of 

 causing the formation of specific antibodies. But we have 

 been obliged to remember that processes so simple and so 

 efficacious as vaccination against anthrax and serum therapy 

 in diphtheria cannot be applied to every infection. 



The action of antigens on the organism including the 

 reactions which they provoke as well as the properties of 

 antibodies and the compounds which they form with their 

 antigens differ widely among themselves and produce many 

 contrary effects. Thus it is known that the injection of 

 bacteria, killed or living, as well as bacterial secretions pro- 

 vokes in the injected animal the formation of antibodies 

 which agglutinate and precipitate these bacteria "in vitro"; 

 that in certain cases (cholera) microbes are destroyed by 



