THE THEORY OF IMMUNITY 131 



rendered the pathogenic cells or toxins harmless. Such a 

 process is obviously exactly similar to those which occur 

 with enzymes acting on any given substrate in the presence 

 of a combining body : so tyrosinase breaks up tyrosin, and 

 causes it to combine with the oxygen yielded by peroxide 

 bodies. A similar process occurs with the ordinary hydro- 

 lytic enzymes, as when fats, carbohydrates, and proteins 

 combine with water in the presence of their specific enzymes, 

 such as steapsin, amylopsin, and trypsin. It may, how- 

 ever, be further suggested that all these actions are 

 really immunizing actions, and that, instead of immune 

 bodies being classed among enzymes, the latter should be 

 classed generally among immune bodies, and both among 

 catalysts, the difference between the first two classes 

 being that enzymes dealing with food are a gradual 

 result of evolution, and that what are usually called 

 immune bodies are special ad hoc reaction complexes 

 of a similar order depending on the general powers of 

 reaction in the body tissues. 



It is obvious that foods when not broken up are either 

 poisons or something which cannot be used, and must 

 be excreted. Thus proteins injected into the blood-stream 

 are hemolytic. To be endured or used they must be 

 broken up into animo-acids. What particular quality it 

 is in them which makes them " antigens " is obscure ; 

 but it will certainly be found eventually that it is due 

 in all cases to their " poisonous " (i.e. disturbing) action, 

 since they are wholly out of their evolutionary place. 

 It is said that there is no absolute relation between toxicity 

 and defensive reaction ; but this is only to state the obvious 

 fact that the organism is not armed at all points, and may 

 be destroyed before it can react, or that it is already 

 supplied with general immune catalysts which deal easily 



