FACTS AND PROBLEMS OF IMMUNITY 311 



up by phagocytic cells. The complement is not increased during 

 immunization. 



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 phago- 

 cytes 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 

 has also been shown that many cells in different parts of the body may 

 take part in phagocytosis both under conditions of normal physiological 

 processes and under the stress of invasion. The recent work of Kyes 1 

 has inclined us to think that phagocytosis by fixed tissue cells plays a 

 more important part in protection than hitherto suspected*. Kyes has 

 found that pigeons which possess a powerful normal resistance against 

 pneumococci, dispose of these bacteria by phagocytosis carried on in 

 the liver by endothelial and other cells. Injected either intraperitoneally 

 or intravenously, the bacteria are soon found collected in large masses 

 within such cells. 



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 entering 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 leucocytes 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 



l Kyes, Jour, of Infect. Dis., March, 1916. 



