192 PHAGOCYTOSIS 



Phagocytosis in Acquired Immunity. Metschnikoff has shown 

 that acquired immunity is largely due to stimulated phagocytosis. 

 This is true whether the immunity be due to one attack of the disease 

 or to vaccination. A rabbit which has been immunized to anthrax 

 shows a more marked phagocytosis on inoculation with a virulent cul- 

 ture than does a fresh animal. In the latter there is only a slight 

 serous exudate at the point of inoculation, while in the former there 

 is an abundant accumulation of leukocytes, which ingest and digest the 

 bacilli. A drop of this exudate injected into an animal highly suscep- 

 tible to anthrax, a guinea-pig or mouse, usually results in fatal infec- 

 tion, showing that the included bacilli are not immediately killed. 

 Some hours after inoculation of an immunized animal, microscopic 

 examination of the exudate shows no free bacilli. All are included in 

 the leukocytes and some seem normal while others show granulation 

 and others degenerative changes. Similar observations have been 

 made on other animals immunized to anthrax by vaccines. The phago- 

 cytes are stimulated by the introduction of the attenuated bacilli of the 

 vaccines, and finally become strong enough to contend successfully 

 with those of full virulence. 



Pfeiffer has shown that if guinea-pigs be highly immunized to the 

 cholera vibrio by repeated intraperitoneal injections, the clear peri- 

 toneal fluid dissolves these organisms much as water dissolves sugar 

 or salt. There is no phagocytosis in evidence. In fact, the peritoneal 

 fluid is quite free from leukocytes. This is known as Pfeiffer's phe- 

 nomenon, and it has been held by many that it overthrew Metschni- 

 koff's theory of the importance of phagocytosis in acquired immunity. 

 In fact, this phenomenon has divided immunologists into two camps, 

 the holders of the humoral, and those of the cellular theory. Metsch- 

 nikoff explains Pfeiffer's phenomenon as follows : The repeated injec- 

 tions of the vibrio into the peritoneal cavity have accumulated in that 

 viscus many phagocytes which have been injured and have given up 

 their contents, including their digestive enzyme, to the surrounding 

 fluid. In fact, Metschnikoff has demonstrated masses of these dead 

 leukocytes on the peritoneal walls and the omentum. This destruction 

 and dissolution of the phagocytes he designates phagolysis. The agent 

 in the fluid which dissolves the vibrios is the enzyme from the injured 

 leukocytes. The solvent action proceeds extracellularly, but the sol- 



