MIRACLES OF SCIENCE 



animals immune to these poisons by giving them 

 repeated small doses of the drug. A conspicuous 

 immediate result of these experiments was the de- 

 velopment (about 1897) of Ehrlich's theory of im- 

 munity, which may be regarded as the most plaus- 

 ible hypothesis hitherto advanced in explanation of 

 the observed facts of immunity to bacterial diseases. 

 Ehrlich called his explanation the side-chain theory. 



This very interesting hypothesis, although based 

 on a vast range of observations, is of necessity alto- 

 gether theoretical. But it has proved of great im- 

 portance in enabling workers in many fields to visu- 

 alize what at least are the possible conditions in the 

 human system through which it comes to pass that 

 the person who has recovered from a bacterial dis- 

 ease is very generally immune for a time at least to 

 further attacks from that disease; and also the allied 

 fact that an antitoxin, developed in the system of an 

 animal, may be transferred with curative effect to 

 the system of a patient suffering from the bacterial 

 toxin, as illustrated in the familiar case of the Von 

 Behring cure for diphtheria. 



The side-chain theory in its fully developed form, 

 as expounded, for example, by Ehrlich before the 

 Royal Society of London in his Croonian lecture, in 

 1900, is a very elaborate hypothesis. It assumes 

 that the toxin produced by a pathogenic bacterium, 

 the poisonous effects of which produce the symptoms 

 of disease, consists essentially of ultra-microscopic 

 particles which may be conceived as having definite 

 forms, varying with different types of bacteria, but 

 uniform in the case of any given toxin. 



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