224 MECHANISM OF INFECTION AND IMMUNITY 



hydrates, fats and extractives, but the functions of life reside in its 

 protein molecules. Each kind of life must consist of its own specific 

 proteins and there are as many kinds of proteins as there are kinds 

 of cells. It follows that proteins are specific. Those of the colon 

 bacillus are not identical with those of the typhoid and differ more 

 widely still from those of the tubercle bacillus. Relationship between 

 varieties and species depends on similarity in the chemical constitution 

 of the molecules. The essential proteins of wheat and barley are not 

 identical, but are more closely related in chemical structure than are 

 those of barley and those of pumpkin seed. 



All cells, so long as they are in active life, must feed. Otherwise, 

 they cannot grow and multiply. This is equally true of cells which 

 have an individual existence and constitute unicellular forms of life, 

 and of those which have a communal life and exist in the organs of 

 multicellular beings, such as man. A living cell can feed only on 

 that with which it comes in contact. Some of the cells of man's body, 

 such as the leukocytes, can go in quest of food, while others are fixed 

 and must depend on what is brought to them. 



Each cell feeds by means of its enzymes which split up the pabulum 

 into blocks which can be fitted into its molecular structures. Each kind 

 of cell must have its own specific ferment or ferments and there are 

 as many kinds of ferments or enzymes as there are kinds of cells. 

 There are enzymes which split up carbohydrates, known as* diastases, 

 and those which split up fats, known as lipases, but in our studies 

 of infection and immunity, we are especially concerned with those 

 that split up proteins, known as proteases or proteolytic enzymes or 

 ferments. These enzymes are specific in two senses; first they are 

 products of specific cells and second, they can act only on proteins of 

 certain chemical structure. It must be evident that a cell can feed only 

 on that material which is digestible by its enzymes. This is true of 

 single cells and of multiple cells. Horn contains proteins and other 

 nitrogenous substances, but man cannot live on it because the enzymes 

 of his alimentary canal cannot digest it. Only that which its enzymes 

 can properly prepare for assimiliation is food for the organism, 

 whether it be unicellular or multicellular. With this understanding 

 of the conditions under which cells grow and multiply, we are ready 



