THE SIDE CHAIN THEORY 105 



and antitoxins has received the greatest amount of attention, for 

 here we are dealing with substances which react in solution and whose 

 behavior is thus more readily open to investigation and interpreta- 

 tion than in the case of those phenomena which occur between highly 

 complex components, such as animal and vegetable cells and their 

 antibodies. It may be said in advance, however, that notwithstand- 

 ing many observations which at first tended to suggest that the 

 character of the interaction between toxins and antitoxins was of a 

 different nature than that between the other antibodies and their 

 antigens, more detailed investigations have shown that this difference 

 is more in appearance than in fact. 



A number of modern investigators have attempted to explain 

 the laws which have been found to govern the action of antibodies 

 upon their antigens upon a purely physical basis, but, although 

 many observations may be interpreted as supporting their view, one 

 factor is not explained upon these grounds, and that is the remarkable 

 specificity of the antibodies. Erhlich's side chain theory, on the 

 other hand, which rests upon a purely chemical interpretation of the 

 phenomena of antigen antibody interaction, satisfactorily accounts 

 for this, so that, even though we admit the validity of the physical 

 theory in the explanation of certain phenomena we must still adhere 

 to the chemical side. All the facts which have been observed when 

 toxins and antitoxins interact can certainly be explained upon 

 chemical grounds. In the case of the other antigens and their 

 antibodies, we may admit that physical processes may play a role, 

 but in addition to these, chemical action unquestionably also takes 

 place. A somewhat more detailed account of certain studies along 

 these lines will serve to bring out some of the difficulties which have 

 been encountered. 



As I have pointed out, if we conceive that toxin and antitoxin 

 unite with one another chemically, then we would expect that 

 definite quantities of the one or multiples thereof would unite 

 with corresponding quantities or multiples of the other. To use a 

 common example if 40 parts by weight of sodium hydrate unite with 

 36.5 parts by weight of hydrochloric acid, according to the equation: 



NaOH + HC1 = NaCl + H 2 O 

 m. weight, 40 m. w., 36.5 



then 2 X 40 parts of NaOH will unite with 2 X 36.5 parts of HC1, 

 and 3 X 40NaOH with 3 X 36.5 HC1, etc. 



