IMMUNITY TO TOXINS 135 



production of tetanus. Taking these two experiments together, 

 we may regard them as constituting a definite proof of the existence 

 of this form of immunity. The theory that the toxin disappears 

 because it is anchored to the cells appears to be clearly demon- 

 strated from the fact that antitoxin is produced, an occurrence 

 which we could scarcely explain on any other hypothesis. 



But it is hardly necessary to look for experimental proof of the 

 occurrence of this form of immunity, since the fact that the blood 

 of many species of animals is toxic for other species appears to 

 constitute a ready-made demonstration of the fact. The most 

 striking example is given by eel serum, a substance which is toxic 

 for nearly all animals, except, of course, the eel itself. The serum 

 of the horse is perhaps the least toxic of all sera, but even this 

 has some poisonous properties. Whatever theory we may hold 

 with regard to the mechanism by which the cells of an animal 

 are nourished, it is hardly possible to avoid the conclusion that the 

 proteid substances of the plasma must combine with the living 

 protoplasm. And it is the proteids which make eel serum toxic 

 to other animals. It appears, therefore, that the immunity of the 

 eel to its own serum depends upon the fact that its protoplasm is 

 not injured by the toxophore group, although the molecules of the 

 toxin and protoplasm become united. (The toxicity of eel serum 

 is dependent on a more complex structure than that of ordinary 

 bacterial toxins, being, in fact, a cytolysin or cytotoxin, but this 

 does not affect the argument.) 



A consideration of the use of these poisonous sera to the 

 animals which produce them may perhaps enable us to analyze 

 the process a step further, and to attribute their immunity (e.g., 

 of eels to eel serum) to the power which their cells possess of 

 using these "toxic" substances as sources of nourishment; and 

 if this be so, we might perhaps apply this suggestion to explain 

 the form of immunity to toxins under discussion. Thus, in the 

 case of the alligator injected with tetanus toxin and kept in the 

 cold, it is possible that the poison which combines with the cells 

 is " digested " by them and used as nourishment. If this is so, we 

 must assume that the molecule of toxin has, as far as its hapto- 

 phore group is concerned, a close chemical affinity with the 

 proteids normally present in the animal's blood and used by it in 

 cell nutrition ; and, further, that the toxophore group is not only 

 innocuous to the cell, but is also no bar to the assimilation of the 

 whole molecule by it. We have seen that in the case of the frog 



