CHAPTER XII 



THE EELATION OF THE LEUKOCYTES AND OF 

 PHAGOCYTOSIS TO IMMUNITY 



IN Metchnikoff's earliest work upon the daphnia or water flea 

 he observed clearly that there was a direct relation between the de- 

 gree of phagocytosis and the outcome of the infection. When 

 phagocytosis of the invading yeasts was energetic and complete the 

 daphnia recovered. When the yeast cells penetrated the intestinal 

 wall of the daphnia in large numbers, and were enabled to multiply 

 before the phagocytic cells could accumulate in sufficient numbers to 

 engulf them, then the body of the daphnia was soon swamped with 

 the parasites and death ensued. 



This simple observation fostered the thought that the basic prin- 

 ciple underlying all processes of immunity was represented in this 

 struggle between the invading bacteria and the phagocytic cells. 

 To the activity of the latter, entirely, he attributed the power of 

 resistance. 



In support of this contention Metchnikoff and his pupils have 

 marshaled many facts, most of which are set forth in his classical 

 work "L'Immunite dans les Maladies Infectieuses." It will be 

 manifestly impossible here to do more than outline the plan of 

 study which these investigations have followed and the conclusions 

 to which they gave just foundation. 



The original study upon the infectious disease of daphnia led to 

 analogous experiments upon higher animals and, by the prolonged 

 and patient investigations of Metchnikoff and his pupils, it was 

 shown that, throughout the field of infectious disease, there was a 

 striking parallelism between the resistance of the infected subject 

 and the degree of phagocytosis which occurred. 



Earlier studies concern themselves chiefly with the natural im- 

 munity possessed by many animals against certain infection. The 

 infectious disease which at this time had been most thoroughly 

 studied was anthrax, and Koch had shown that frogs and other cold- 

 blooded animals were markedly resistant against this micro-organism. 

 Taking advantage of this observation, Metchnikoff studied the phago- 

 cytosis of anthrax bacilli in frogs and found that it took place rap- 

 idly and effectively, all of the injected bacilli being soon engulfed 

 by the accumulating cells. Similarly, active phagocytosis of anthrax 



