586 IMMUNITY 



anti-substance enters into combination with its corresponding 

 substance antigen. The dual constitution of toxins and kindred 

 substances, as already described (p. 195), is also of importance in 

 this connection. Now, to take the case of toxins, when these 

 are introduced into the system they are fixed, like food-stuffs, 

 by their haptophorous groups to the receptors of the cell 

 protoplasm, but are unsuitable for assimilation. . 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 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 R.-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 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 did 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 



