MECHANISM OF PROTEIN HYDROLYSIS AND IMMUNIZATION 33 



grams have any representation in the world of fact. They are 

 figments of the imagination, and may serve some such useful 

 purpose as picture blocks serve in teaching a child the alphabet. 

 But as the time comes when the child puts aside the blocks and 

 takes in hand the pen, so pathologists must ultimately lay aside 

 the crude mechanism of haptophores and amboceptors and learn 

 to deal with the phenomena of immunity in terms of the protein 

 molecule and the chemical atom. 



PROTEIN METABOLISM 



To be sure the chemistry of the protein molecule is by no 

 means as clearly understood as might be desired, even by spe- 

 cialists in biochemistry. But the researches of many workers 

 in recent years have resulted in tearing the molecule apart in 

 the laboratory, and in revealing the major part of its primary 

 constituents. We are now gaining an inkling as to what really 

 happens when proteid foods are taken into the digestive tracts 

 and subjected to the digestive ferments. And we are beginning 

 to realize that the subject has supreme importance from the 

 standpoint of the student of infectious diseases, for the simple 

 but all-sufficient reason that the microbic agents that cause these 

 diseases are themselves protoplasmic bodies that is to say, com- 

 pounds of protein. I shall argue presently that the bacterial 

 proteins are of a relatively unevolved type, comparable perhaps 

 in complexity to peptones rather than to full-sized molecules of 

 the proteins of higher plants and of animals; but in any event 

 the difference is one of degree only. The bacterial substance, 

 within its lipoid membrane, is a nitrogenous or proteid body. 



When we reflect that there are always myriads of these pro- 

 teid bodies in the digestive tract; and that legions of them on 

 occasion find their way into the vascular system, and are there 

 digested, the pertinence of the topic, in relation to protein meta- 

 bolism, becomes evident. 



And from the present standpoint the chief interest centers 

 on the fact that there is in the human body one set of cells and 

 one only that has been demonstrated to be able to digest and 

 metamorphose the bacterial proteins when once they have invaded 

 the blood stream namely the leucocytes. What the ferments 

 of the digestive tract accomplish in the case of the food pro- 

 teins, is accomplished by ferments of the leucocyte in the case 

 of the bacterial proteins with which it comes in contact. 



I shall suggest that the function of the leucocyte in this capac- 

 ity is far more general, having to do with the metamorphosis 

 of many types of protein in addition to those that come with the 

 bacteria; but for the moment it suffices to call attention to the 



