376 Chapter XII 



of toxin to obtain the same rise of temperature. Quite recently, 

 having now lost the greater part of its humoral antitoxic power, 

 this horse exhibited no rise of temperature after an injection of 

 250 C.C. of strong diphtheria toxin. The diminution of the specific 

 susceptibility is produced in this case in a most marked fashion ; it 

 is not therefore to the antitoxic property of the body fluids that 

 this case of immunity must be attributed. 



The insusceptibility acquired against poisons of different kinds 

 is observed also in cases where the adaptation is not accompanied 

 [395] by the production of humoral antitoxic properties, as in the im- 

 munity of frogs against abrin. This form of immunity may be traced 

 through the organic series down to such lowly developed organisms 

 as the Plasmodium of the Myxomycetes, which as we have seen 

 readily becomes adapted to different poisons (see Chapter II). 



It can be clearly seen, then, that immunity against toxic sub- 

 stances is a very complex phenomenon which it is impossible to 

 reduce simply to an antitoxic function of the fluids of the body. 

 For this reason we cannot accept a theory which would confine this 

 kind of immunity within the narrow limits of a simple reaction 

 between two substances, a reaction quite comparable to that observed 

 in a test-tube. Attempts have been made to determine with almost 

 mathematical precision the conditions under which it is possible to 

 communicate to the animal a resistance against microbial toxins and 

 formulae have been constructed to define these conditions. But 

 the application of these formulae has been found to be a much more 

 difficult matter. In Prussia, with the sanction of the Government, 

 regulations have been enacted as to the procedure to be followed 

 in the testing of antitoxic serums, and a paragraph has been added 

 which requires a j)ost-mortem examination of the guinea-pigs em- 

 ployed for this purpose in the case of diphtheria antitoxin. "The 

 dead animals," says this instruction, "must be submitted to a post- 

 mortem examination, and special attention must be directed to the 

 presence of any pre-existing diseases (tuberculosis, pseudotuber- 

 culosis, pneumonia) which may have induced hypersusceptibility in 

 the animals under experiment." Do we not see in this a proof of 

 the important intervention of the organism of the living animal 

 which may modify the results of calculations based upon too rigorous 

 formulae? It must not be forgotten, too, that in addition to the 

 three diseases named in the instructions, we have a number of other 

 factors which may influence the receptivity and the resistance of 



