1 56 IMMUNO-CATALYSIS 



have considered antigens as iF they were analogous to the substrates in 

 enzyme reactions. This is to some extent justifiable as relatively small 

 molecules can act as substrates and combine with antibodies; whereas 

 antibodies and apparently all enzymes consist of large molecules. It is 

 also convenient since the effect on specificity of chemical changes in 

 antigen can be compared with the effect of similar changes in sub- 

 strates. But it may be more correct to regard antigens, whose action 

 is to produce some change in antibodies that results in insolubility, as 

 analogous to enzymes." 



For the sake of historical interest, the concept presented by Wells 

 (1929) in his book on the Chemical Aspects of Immunity may be 

 briefly mentioned at this point. After summarizing the known facts 

 regarding the resemblances of the physico-chemical properties of toxins 

 and enzymes, Wells writes as follows : 



"Enzymes and toxins seem to produce their effects according to 

 different laws:— A small amount of enzyme can in course of time pro- 

 duce an almost indefinite amount of effect, whereas toxins act more 

 nearly quantitatively.* It seems as if the enzyme were bound to the 

 body upon which it acts, as is the toxin, but that after it has destroyed 

 this body it is set free in a still active form ready to accomplish further 

 work; whereas the toxin is either not set free, or it becomes inactive 

 after it has once been combined." 



4. Immune and Enzyme Reactions— A Comparison of 

 Reaction Mechanisms 



The comparison of the two systems can be schematically dia- 

 grammed as follows: 



Antigen -j-globulin factorsf -> Antibody globulin 

 Enzyme+substrate ^» Reaction products 



It is apparent from the above schema that in the in vivo mechanism 

 the following pairs have the same roles: 



(a) Antigen and Enzyme 



(b) Globulin Factorsf and Substrate 



(c) Antibodies and Reaction Products 



^Combination between toxin and antitoxin. 



fThe term Globulin Factors does not stand for normal globulins. It stands for 

 peptides or amino acids which are utilized for the synthesis of antibody globulins. 



