134 NATURAL IMMUNITY TO TOXINS 



toxophore groups. We have already seen reason to believe that 

 the former can functionate (i.e., the molecule of toxin can unite 

 with the cell) at a low temperature, whereas the enzyme-like 

 action of the toxophore radical can only act at the temperature of 

 the human body, or one approximating thereto this is in the case 

 of tetanus. It is apparent, therefore, at the outset that there are 

 two possible explanations for immunity to toxins : firstly, the 

 toxin may find no cell with which it can unite or, to use Ehrlich's 

 terminology, no cell receptor having a combining affinity for the 

 haptophore molecule of the toxin molecule ; and, secondly, the 

 union takes place, but the cell is not injured as a consequence 

 that is, the toxophore group is without action on the cell proto- 

 plasm. 



1. Immunity due to an absence of suitable receptors. 



This has been proved to exist in several cases. Most of these 

 are due to the researches of Metchnikoff, who, however, does not 

 express the fact in these words, but contents himself by saying 

 that the toxin does not combine with the tissues. Thus, when the 

 larva of Oryctes is injected with tetanus toxin, no symptoms 

 develop, and if the blood of the animal be collected several months 

 afterwards, it will be found to contain free toxin, as shown by the 

 fact that it will tetanize susceptible animals. These experiments 

 were carried out at a temperature of 30 to 36 C. Similar facts 

 were observed in lizards ; Jjy c.c. of blood taken two months after 

 the injection of tetanus toxin produced fatal tetanus in a mouse ; 

 the blood of a turtle (Emys orbicularis) contained toxin no less 

 than four months after injection. In these cases the explanation of 

 the immunity is clear, however we choose to express it : the toxin 

 has no chemical affinity for the living protoplasm of any part of 

 the animal, and is, therefore, powerless to injure it. 



2. Immunity due to the insusceptibility of the cells to the 

 action of the toxophore group. 



Perhaps the best example of this is in the case, so often referred 

 to, of the action of tetanus toxin on the cold frog, but other 

 striking examples have been discovered by Metchnikoff. Thus, 

 when the alligator is kept at ordinary temperatures (about 20 C.) 

 and injected with tetanus toxin, that substance rapidly disappears 

 from the blood, yet without the production of any tetanic symptoms, 

 and without the appearance of antitoxin in the blood. If, how- 

 ever, the animal is kept at a temperature of 32 to 37 C., it pro- 

 duces antitoxin with great rapidity, though still without the 



