EHRLICH'S SIDE-CHAIN THEORY. 491 



facts of passive immunity, it must be kept in view that all the 

 substances for which anti-substances have been obtained are, like 

 proteids, of unknown but undoubtedly of very complex chemical 

 constitution, and that in apparently every case the anti-substance 

 enters into combination with its corresponding substance. The 

 dual constitution of toxins and kindred substances, as already 

 described (p. 477), is also of importance in this connection. 

 Now when toxins are introduced into the system they are fixed, 

 like food-stuffs, by their haptophorous groups to the receptors of 

 the cell protoplasm. If they are in sufficiently large amount the 

 toxophorous part of the toxin molecule produces that disturbance 

 of the protoplasm which 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 combination of receptors with toxin is supposed to 

 be of firm nature, the receptors are lost for the purposes of the 

 cell, and the combination R.-T. (receptor -1- 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 consti- 

 tute antitoxin molecules. So that 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 the toxin can 

 no longer combine with the cells and exert a pathogenic action. 

 Antitoxin nioleades are thus free receptors of the first order. A 

 corresponding explanation applies to the origin of antibacterial 

 and like sera. The molecules of bacterial bodies, of stromata of 

 red corpuscles, etc., act as unsuitable food-stuffs to the cells and 

 use up the receptors which combine with them. These molecules 

 are chemically of larger size than the toxin molecules, and the 

 corresponding receptors are those which can also fix a ferment. 

 The im^nune bodies of antibacterial, hcBmolytic, and other like_ 

 sera are thus free receptors of the second order. 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 



