EHRLICH'S SIDE-CHAIN THEORY 587 



blood. The receptors thus lost become replaced by new ones, 

 and when additional toxin molecules are introduced, these new 

 receptors are used up in the same manner as before. As a result of 

 this repeated loss, the regeneration of the receptors becomes an 

 over-regeneration, and the receptors formed in excess appear in 

 the free condition in the blood stream and then constitute anti- 

 toxin molecules. There are thus three factors in the process, 

 namely, (1) fixation of toxin, (2) over-production of receptors, (3) 

 setting free of receptors produced in excess. Accordingly these 

 receptors which, when forming part of the cell protoplasm, anchor 

 the toxin to the cell, and thus are essential to the occurrence of 

 toxic phenomena, in the free condition unite with the toxin, and 

 thus prevent the toxin from combining with the cells and exert- 

 ing a pathogenic action. The three orders of receptors, when 

 separated from the cells, thus give the three kinds of anti- 

 substances. Ehrlich does not state what cells are specially 

 concerned in the production of an ti -substances, but from what 

 has been stated it is manifest that any cell which fixes a toxin 

 molecule, for example, is potentially a source of antitoxin. 

 Cells, to whose disturbance, resulting from the fixation of toxin, 

 characteristic symptoms of poisoning are due, will thus be 

 sources of antitoxin, e.g., cells of the nervous system in the case 

 of tetanus, though the cells not so seriously affected by toxin 

 fixation may act in the same way. The experimental investiga- 

 tion of the source of antitoxins has, however, yielded little result, 

 and no definite statement can be made on the subject. 



When we come to consider how far Ehrlich's theory is in 

 harmony with known facts, we find that there is much in its 

 favour. In the first place, it explains the difference between 

 active and passive immunity, e.g., difference in duration, etc. ; in 

 the former the cells have acquired the habit of discharging anti- 

 substances, in the latter the anti-substances are simply present 

 as the result of direct transference. It is also in harmony with 

 the action of antitoxins, etc., as detailed above, and especially 

 it affords an explanation of the multiplicity of anti- substances. 

 For, if we take the case of antitoxins, we see that this depends 

 upon the combining affinity of the toxin for certain of the cells 

 of the body, and this again is referred back to the complicated 

 constitution of living protoplasm. Furthermore, the biological 

 principle involved is no new one, being simply that of over- 

 regeneration after loss. It would appear likely that the integrity 

 of the functional centres of the protoplasm molecules would be 

 essential to the satisfactory production of side-chains, and this 

 would appear in accordance with the fact that antitoxin 



