THE ORIGIN OF ANTITOXIN THE SIDE-CHAIN THEORY IOI 



stituents of the red blood-corpuscles, the substances forming their 

 stroma, call forth the production of haemolysins ; and a great many 

 other cases might be quoted. It may be taken as definitely 

 established that wherever the nature of the antigen is known and 

 falls into one of the known groups, that antigen is a proteid. 



We may almost go a step farther, and say that all proteids, of 

 whatever nature, when injected into suitable animals, will give 

 rise to antibodies ; in fact, some writers actually enunciate this 

 as a law. There are, however, a few exceptions, which are 

 gradually diminishing in face of further researches, and it is 

 highly probable that time will show that the law is valid to the 

 fullest extent. 



Reverting now to the question of the nature of the bodies of 

 unknown constitution which form antibodies i.e., the toxins, 

 enzymes, etc. we may ask whether these are not in reality 

 proteids, in spite of their failure to give the proteid reactions as 

 usually accepted by chemists. The question is largely one of 

 nomenclature. Accepting the definitions usually given in physio- 

 logical textbooks, and based on a study of ordinary proteids, such 

 as egg-albumin, etc., we may admit that they are not; but are 

 these definitions valid ? Might we not define them with greater 

 accuracy as substances from which animal organisms can obtain 

 food material containing combined (" organic ") nitrogen ? In 

 this case we should substitute for coarse chemical experiments 

 in vitro the more delicate reactions of the living animal body; 

 any nitrogenous substance which is recognized by living proto- 

 plasm as being a suitable pabulum for it would be defined as a 

 proteid. If this is the case, we are thrown back at once on 

 Ehrlich's theory, and can define the proteids as substances which 

 unite in a specific manner, haptophore to receptor, with the living 

 molecule of protoplasm. If we do this, we may admit that the 

 toxins and enzymes, whatever their chemical reactions, and 

 whether they are or are not food substances for the cells, combine 

 with protoplasm in the manner which is characteristic of proteids, 

 and are to be regarded as proteids when looked at from this point 

 of view. And we must not forget that many of the substances 

 which we regard as toxins are, as a matter of fact, of value in 

 nutrition to the animal which forms them, and only act as poisons 

 in strange species. Thus, the ichthyotoxin of eel serum must 

 nourish the cells of the eel, but is a powerful toxin to almost 

 all other species. Whether any animal can utilize tetanus toxin, 



