588 IMMUNITY 



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. 441) 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 

 introduced. Further, when the serum of an animal contains 

 a large amount of antitoxin, how does the additional toxin 

 injected reach the cells in order to influence them as we know 

 it does 1 This also is difficult to understand, unless the toxin 

 has a greater affinity for the receptors in the cells than for 

 the free receptors (antitoxin) in the serum. A supersensitive- 

 ness of the nerve-cells of an animal to tetanus toxin, sometimes 

 observed even when there is a large amount of antitoxin in 

 the serum, has been often brought forward as an objection. 

 But this also may perhaps be explained by there having occurred 

 a partial damage of the cell protoplasm by the toxic action in 

 the process of immunisation an explanation which, of course, 

 demands that in some way the freshly introduced toxin may 

 reach the cells in spite of the antitoxin in the blood, or it may 

 belong to the group of anaphylactic phenomena described below 

 (p. 595). Further investigation alone will settle these and 



