124 



Immunity 



comes possible for us to attempt to solve the mystery of the 

 origin of these bodies. I have in the beginning of this 

 lecture* fully developed the 'side-chain theory,' according 

 to which the antitoxins are merely certain of the protoplasm 

 side-chain' which have been produced in excess and pushed 

 off into the blood. 



"The toxins as secretion products of the cells are in all 

 likelihood still relatively uncomplicated bodies ; at least by 

 comparison with the primary and complex albumins of which 

 the living cell is composed. 



"If we now recognize that the different lysins arise only 

 through absorption of highly com- 

 plex cell material such as red blood- 

 corpuscles or bacteria then the ex- 

 planation, in accordance with what 

 I have said, is that there are present 

 in the organism 'side-chains' of a 

 special nature, so constituted that 

 they are endowed not only with an 

 atomic group by virtue of the affini- 

 ties of which they are enabled to pick 

 up material, but also with a second 

 atomic group, which, being ferment- 

 loving in its nature, brings about the 

 digestion of the material taken up. 

 Should the pushing off of these ' side- 

 chains ' be forced, as it were, by im- 

 munization, then the 'side-chains' 

 thus set free must possess both groups, 

 and will, therefore, in their character- 

 istics entirely correspond with what 



we have placed beyond doubt as regards the 'immune 

 body' of the hemolysin." 



An analysis of this theory shows complete natural im- 

 munity to depend upon the absence of haptophore groups 

 (receptors) by which the toxins can be united to the cells. 

 Extreme sensitivity or susceptibility probably depends 

 upon the adapted haptophores being present or at least 

 most numerous upon the cells of highly vital organs; com- 

 parative insensitivity or insusceptibility upon the fact that 



* These lengthy extracts, through which I have endeavored to 

 enable Ehrlich to explain his theory to the reader, are taken from his 

 " Croonian Lecture," delivered before the Royal Society of London, 

 March 22, 1900. 



Fig. 1 6. Cell with 

 receptors of the second 

 order (a) by which the 

 cells fix useful mole- 

 cules, of albumins, etc., 

 on one hand (6), and zy- 

 mogen molecules (c) on 

 the other hand, and 

 make use of the one sub- 

 stance through the ac- 

 tion of the other. 



