CHAPTER XVII 



PHAGOCYTOSIS 



The studies on immunity which we have outlined in the preceding 

 sections have dealt entirely with the phenomena occurring in the re- 

 action 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 con- 

 cerning 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 

 Metchnikoff' 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 conceptions 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 wiU be again extruded after a 

 varying period. If, however, the ingested substance is of a nature which 

 can be utiHzed in the nutrition of the protozoon, 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^ which acts in the 



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

 ' Mouton, Ann. de I'inst. Pasteur, xvi, 1902. 

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