EHRLICH'S SIDE-CHAIN THEORY 587 



active and passive immunity, e.ij., 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 

 formation occurs most satisfactorily when there is no marked 

 disturbance of the health of the animal. 



It is to be noted, however, that it does not explain active 

 immunity apart from the presence of anti-substances in the 

 serum. For example, an animal may be able to withstand a 

 much larger amount of toxin than could be neutralised by the 

 total amount of antitoxin in its serum. This might theoretically 

 be explained by supposing a special looseness of the cell re- 

 ceptors so that the toxin-receptor combination became readily cast 

 off. The question, however, arises whether there may not be 

 really an increased resistance of the cells to the toxophorous 

 actions. An observation made by Meyer and Ransom {vide 

 p. 432) is also difficult of explanation, according to the view 

 that antitoxin is formed by the cells with which the toxin 

 combines and on which it acts. They found that in an animal 

 actively immunised against tetanus and with antitoxin beginning 

 to appear in its blood, the injection of a single M.L.D. of 

 tetanus toxin into a peripheral nerve brought about tetanus 

 with a fatal result. On the other hand, the injection of anti- 

 toxin into the sciatic nerve above the point of injection of toxin 

 prevented the latter from reaching the cells of the cord. One 

 can scarcely imagine an explanation of these facts if antitoxin 

 molecules were in process of being shed off by the cells of the 

 nervous system. There is also the fact, very difficult of explana- 

 tion according to the theory of regeneration of receptors, or, 

 indeed, according to any theory, that the amount of anti- 

 substance produced, as tested by its combining equivalent, may 

 be many times what would correspond to the amount of antigen 

 injected. Further, when the serum of an animal contains 



