44 IMMUNITY 



into sugar. They are not directly concerned in the nutrition of the 

 organism. Cells of a protecting character in man are either micro- 

 phages, or macrophages. The microphages are the polynuclear 

 leucocytes, which are concerned in the protection of the organism 

 against acute infections, the bacteria of which they take up and 

 devour. The macrophages consist of the large lymphocytes, the 

 endothelial cells, and some connective tissue cells, which take up 

 foreign bodies. Both of these classes contain ferments; micro- 

 cytase being found in the microphages; and macrocytase in the 

 macrophages. The latter absorb connective tissue cells through 

 their particular ferments, and are active in immunizing against 

 tuberculosis. These cells perform various functions in the body. 

 When the tissues are invaded with bacteria, the blood shows an 

 increase in the number of these microphages, which have been 

 called the "hygienic police." Summoned to repel invasion, they 

 leave the lymph stream for that of the blood. All the phenomena of 

 leucocytic emigration in inflammation is a manifestation of positive 

 chemotaxis. During practically all the infections, the peripheral 

 blood contains an excess of leucocytes over the normal amount per 

 cubic millimeter (7,600). In exceptional infections, typhoid fever, 

 influenza, measles, and tuberculosis, there is no such increase, or 

 leucocytosis. In malaria (not a bacterial infection) there is also 

 no leucocytosis. 



Metchnikoff has described a process in which the phagocytes 

 undergo what he calls phagolysis. The ferment, cytase, is dis- 

 charged and acts extracellularly, as in the haemolysis of foreign red 

 blood cells in the peritoneum of a guinea pig. This phagolysis in 

 dissolving of the leucocyte is the cause of the chemical phenomena 

 (so he avers) which cannot be ascribed merely to phagocytosis. 

 MetchnikofI further claims that both phagocytosis and phagolysis, 

 either severally, or in combination, are responsible for natural or 

 acquired immunity. 



In the case of acquired immunity, it is supposed that the leuco- 

 cytes become educated. Regarding the toxins against which 



