188 PHAGOCYTOSIS 



It must be evident from the facts already stated that it is a func- 

 tion of the white blood corpuscles, to fall on invading cells, absorb 

 and digest them. By chemotaxis the phagocytes are called to the point 

 of bacterial invasion. The army of defense is mobilized by the blood 

 and lymph. The individual leukocytes may pass through tissues, 

 including the walls of vessels, and this aids in the movements of con- 

 centration. When the foreign protein, which has found its way into 

 the body, is dead, the only question about it is the ability of the phago- 

 cytes to digest it. If they are not able to do so at once, they may acquire 

 this function. When the foreign protein consists of living organisms, 

 there is a struggle between defenders and invaders. The destruction 

 of the latter does not depend solely on the ability of the former to 

 engulf and digest their opponents. The digestive ferments may be 

 excreted or poured out into the surrounding fluid by the phagocytes, 

 and at their death and dissolution all the ferment contained within 

 their bodies is disseminated throughout the adjacent tissue and exerts 

 its destructive action on the invaders. It seems that in giving their 

 lives for the protection of the body as a whole, the leukocytes may 

 become most effective. The secretions from the phagocytes are con- 

 stantly, in health and in disease, passing into the blood, and, besides, 

 these cells are constantly suffering death and dissolution and in this 

 way blood and serum receive a constant supply of germicidal enzymes. 



Besides the leukocytes, there are certain fixed cells which may be 

 regarded as phagocytes. Such are connective tissue and endothelial 

 cells. However, some of these are modified leukocytes. For instance, 

 the Kupffer cells of the liver are now regarded by Metchnikoff as large 

 mononuclear leukocytes. This leaves the large cells of the lymph 

 glands, the spleen and bone marrow as fixed phagocytes. These are 

 regarded as playing only a subordinate role in immunity when com- 

 pared with the wandering leukocytes. 



Phagocytosis in Natural Immunity. MetchnikofFs attention was 

 called to the subject of phagocytosis by observations on an infection 

 in a small, fresh-water crustacean, the daphnia, which on account of 

 its transparency offers special opportunity for a study of this kind. 

 This organism is infected by the spores of a certain yeast, which gain 

 access to the body cavity, where, when unhindered, they multiply in 

 great numbers and soon destroy their host. However, the invader 



