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Dr. Paul Ehrlich. 



of the normal horse certain substances may be present which possess 

 side-chain affinities similar to those of the diphtheria toxine, and 

 which, therefore, are quite as capable as the latter are of taking posses- 

 sion of the cell side-chains, and occasioning the regeneration and 

 pushing off of these from the cells ; in other words, of causing the 

 presence of an actual diphtheria antitoxine in a normal animal. 



Such occurrences direct attention to the possibility of producing 

 immunity in some cases by the administration of definite food-stuffs. 

 Perhaps we have in some such peculiarity of feeding and tissue- 

 change the explanation of the fact so difficult to understand, viz., 

 that individuals of the same race and species react in such diverse 

 manners to the same infection. Certainly we are very far removed 

 from the solution of this important question, which, as yet, has scarcely 

 assumed a tangible form. Still it is our duty to strive with tenacity 

 to overcome the difficulties which surround this point, bearing in 

 mind the words of your illustrious countryman, Francis Bacon : " Sunt 

 certe ignavi regionum exploratores, qui, ubi nil nisi coelum et pontus 

 videtur, terras ultra esse prorsus negant." 



I have now laid before you the fundamental facts which up to the 

 present constitute our knowledge in the field pertaining to immunity, 

 and which can be most easily and successfully explained through the 

 agency of " the side-chain theory." I wish in a few words to dispel 

 some erroneous ideas which have been advanced in opposition to this 

 theory. 



Eoux has shown that very small quantities of tetanus toxine, if 

 injected directly into the brain, cause the death of the animal. Roux 

 assumes that such an occurrence is not compatible with my theory. 

 Roux is of opinion that according to my theory the brain must be 

 quite immune against tetanus toxine, as the toxophile side-chains of the 

 brain-cells must be identical with the antitoxine, and therefore must 

 exercise an immediate protective action. Experiment showing quite 

 the reverse, the theory is overthrown. 



Roux came to this incorrect conception through an erroneous con- 

 ception of antitoxine. The toxophile side-chains of the brain cells 

 draw directly to themselves the toxine molecules, and, according to 

 my theory, are thus a necessary preliminary condition of the illness. 

 The toxophile groups are therefore really inducers of the action of the 

 poison, and not its preventives. 



Those toxophile groups which, like the antitoxines present in the 

 serum, are able to lay hold of toxine immediately on its entry into the 

 blood, and so to divert it from organs essential to life, can alone be 

 regarded as being possessed of any antitoxic action in the true sense 

 of the word. I may be allowed to call to mind Weigert's excellent 

 simile of iron and the lightning conductor. Iron attracts electricity, 



