CHEMICAL MESSENGERS 73 



action of which has already been described (see p. 57). They 

 appear to be present in all cells, and in most cases the ac- 

 tivity of the cell itself depends upon them.^ These enzymes 

 are very probably of a protein nature and are readily destroyed 

 by heat in the presence of water. The active agents of the 

 external secretions when present are always of the nature of a 

 ferment or enzyme. Driesch- has suggested that the nucleus 

 of the cell is a storehouse of these ferments which pass out 

 into the protoplasm tissues and there set up specific activities. 



Third: Antigens, antibodies including the agents of immunity.^ 

 The active and inactive protein compounds termed antigens in- 

 clude certain known proteins and possibly a few other com- 

 pounds of kindred nature. Among the active protein compounds 

 are certain enzymes, bacterial poisons, snake venoms, spider 

 poisons, and some vegetable poisons; antigens of this class are 

 all powerfully active and possess properties which suggest that 

 they may eventually be classed as enzymes. On the invasion 

 of an organism by any foreign protein of this class in any 

 region except the interior of the alimentary canal it would 

 seem that certain chemical messengers called antibodies arise 

 which are especially fitted to protect the tissues of the body 

 against such invasion; these antibodies are true agents of im- 

 munity and serve to increase the resistance of the organism to 

 any future attack of the invading antigen; it is to this forma- 

 tion of neutralizing antibodies, known as antitoxins, that the 

 curative powers for such infections as diphtheria and tetanus 

 are due. 



There are also antigens of another kind, consisting of inac- 

 tive protein compounds, which, when they invade an organism, 

 induce the formation of antibodies acting in an entirely dif- 



1 Schafer, Sir Edward A., 1916, pp. 4, 5. "Wilson, lidniund B., 1906, p. 427. 



'Zinsser, Hans, 1915, pp. 223-226, 247, 24S. 



