FACTS AND PROBLEMS OF IMMUNITY 311 



bodies is not known to be increased during immunization, at least so 

 far as its presence in the serum is concerned. 



These facts we have learned from the study of the serum; on the 

 other hand, the morphologic investigations instigated and carried on 

 largely by Metchnikoff and his followers have taught us the great part 

 which the formed elements of the blood and lymph play in the protec- 

 tion against and cure of germ diseases, and the importance of the poly- 

 morphonuclear and large mononuclear leucocytes as phagocytes is now 

 widely recognized. 



Of these cells, the polymorphonuclear leucocytes take a very active 

 part in the ingestion and destruction of bacteria, while the large mono- 

 nuclear leucocytes and endothelial cells, especially those lining the blood 

 vessels and body cavities, although also able to ingest bacteria directly, 

 are chiefly active in taking up cells of animal origin, principally those 

 which necessarily, in the normal course of events, belong to the same 

 animal and have probably become injured or have suffered death. 



It does not seem, in this connection, a far-fetched idea to suppose 

 that phagocytic cells may use naturally other cells and bacteria as a 

 part of their regular food supply. The polymorphonuclear leucocytes 

 may thus depend to some extent on the ever-entering bacteria and their 

 remains; for, as we know, bacteria are constantly enteriag along the 

 regular channels of absorption; and it is just as obvious that numbers 

 of blood and tissue cells are constantly dying out and must be disposed 

 of, for such processes are always in evidence in the spleen, and the inges- 

 tion of polymorphonuclear leucocjrtes by the large mononuclears can be 

 observed wherever leucocytes are collected in exudates, due either to 

 infections, poisons, or supposedly benign irritants. The simple fact 

 that these cells retain the basic physiologic activities and an ability to 

 ingest and digest food in its crudest form, which ability was the heritage 

 of their free-swimming ancestors, and that they have not suffered the 

 total specialization and physiologic degeneracy of the fixed tissue cells, 

 seems sufficient evidence to warrant the conclusion that they are most 

 active factors in the protection of the specialized internal tissue cells, 

 which control the general metabolism and higher functions of the 

 animal body. It seems worth mentioning that the leucocytes, alone 

 probably of all the true cells of the body, are entirely independent of the 

 nerve control, and are subject only to the stimulation of their chemical 

 and physical environment, and are thus susceptible of adaptation to 

 and capable of subserving various purposes which would be fatal to the 

 duties of cells controlled by the nerve mechanism for the special func- 



