CHAPTER XVII 



PHAGOCYTOSIS 



THE studies on immunity which we have outlined in the preceding 

 sections have dealt entirely with the phenomena occurring in the 

 reaction between bacteria or bacterial products and the body fluids. 

 These studies, we have seen, have formed the basis of a theoretical 

 conception of immunity formulated chiefly by the German school of 

 bacteriologists under the leadership of Ehrlich, Pfeiffer, Kruse, and 

 others. Parallel with these developments, however, investigations on 

 immunity have been carried on which have brought to light many 

 important facts concerning the participation of the cellular elements 

 of the body in its resistance to infectious germs. 



The inspiration for this work and the greater part of the theoretical 

 considerations which have been based upon it, have emanated from 

 Metchnikon 3 1 and his numerous pupils at the Pasteur Institute in 

 Paris. The phenomenon which these observers have studied in great 

 detail and upon the occurrence of which they have based their con- 

 ceptions of immunity, is known as phagocytosis. 



It is well known that among the lowest unicellular animals the 

 nutritive process consists in the ingestion of minute particles of 

 organic matter by the cell. The rhizopods, which may be found and 

 studied in water from stagnant pools or infusions, when observed 

 under the microscope, may be seen to send out short protoplasmic 

 processes, the pseudopodia, by means of which they gradually flow 

 about any foreign particle with which they come in contact. If the 

 ingested particle is of an inorganic nature and indigestible, it will be 

 again extruded after a varying period. If, however, the ingested sub- 

 stance is of a nature which can be utilized in the nutrition of the proto- 

 zoon, it is rapidly surrounded by a small vacuole within which it is 

 gradually dissolved and becomes a part of the cellular protoplasm. 

 This digestion within the unicellular organism is probably due to a 

 proteolytic enzyme 2 which acts in the presence of a weakly alkaline 



^Metchnikoff, "L'lmmunite dans les maladies infectueuses. " 

 2 Mouton, Ann. de Finst. Pasteur, xvi, 1902. 



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