550 IMMUNITY 



is shown by symptoms of poisoning. If, however, they are 

 in smaller dose, as in the early stages of immunisation, fixation 

 to the protoplasm occurs in the same way; and as the com- 

 bination of receptors with toxin is supposed to be of firm 

 nature, the receptors are lost for the purposes of the cell, and 

 the combination K.-T. (receptor + toxin) is shed off into the 

 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 antitoxin 

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

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

 ting free of receptors produced in excess. Accordingly these re- 

 ceptors 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 anti-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 



