180 



ANTITOXIC IMMUNITY. 



ficity of antitoxin, but reveals, also, a special significance in the incubation period, dur- 

 ing which the conservative forces are mustering. We can realize, furthermore, in the 

 light of this hypothesis, how it is that the protection secured in active immunization is 

 less immediate and also why it is more prolonged than in the passive, since in the latter 

 the available antitoxin is limited to the dosage and is not replenished as in active im- 

 munity by the continued cell activities of the affected individual himself. 



The conception of Ehrlich as to the nature of antitoxin is that of the chemist, and 

 carries over to the performances of protoplasm the presumptions upon which chemical 

 reactions in general are conceived and formulated. But this is not an easy matter, since 



FIG. 89. DIAGRAM ILLUSTRATING THK FORMATION* AND ACTION OF ANTITOXIN IN ACCORDANCE: WITH 



EHRLICH'S "SIDE-CHAIN" HYPOTHESIS. 



A, Portion of cell body ; b, receptors combined with cell protoplasm ; c, receptors separated from 

 cell; a, toxin molecules ; ;/, free receptors antitoxin. 



our knowledge of the ultimate phases of protoplasmic metabolism is very incomplete. 

 The physiological chemist presents to us as his final achievement in analysis an ex- 

 tremely elaborate complex which he calls the proteid molecule. This he does not yet 

 venture to formulate. Thus it is that when we attempt to illustrate in graphic fashion 

 our conception of the performances of proteid molecules, either in normal metabolism 

 or in poisoning, we are forced to use the crudest of symbols. The use of such symbols 

 is not without hazard, for these toxins and these receptors are in truth not histological 

 structures, but molecular groups; they are not alone upon the surfaces but through 

 all the mass of the protoplasm. They do not "break " off from the cell, but are set free 

 as are other chemical substances which result from molecular transformations. Thus 

 if one cannot at last translate these uncouth symbols into the nice conceptions of the 

 chemist they will prove but stumbling blocks. 



There has been much discussion of Ehrlich 's hypothesis, and it has withstood many 

 assaults, mostly inspired by misconceptions of the fundamental claims. The scope of 



