30 VACCINE AND SERUM THERAPY 



protozoon, it is rapidly surrounded by a small vacuole, 

 within which it is gradually dissolved and becomes a 

 part of the cellular protoplasm. It is only as we 

 proceed to animals of a much higher organisation 

 that the function of cell ingestion of crude food is 

 entirely removed from the process of general nutrition. 

 Nevertheless, in these animals also the actual in- 

 gestion of foreign particles occurs, but it is now 

 limited entirely to a definite group of cells. 



In the higher animals this function of phagocytosis 

 is limited to the white blood cells of the circulation, 

 or leucocytes, to certain large endothelial cells lining 

 serous cavities and bloodvessels, and to cells of rather 

 an obscure origin which contribute to the formation 

 of giant cells within the tissues. 



A convenient division of phagocytic cells is that 

 into " wandering " and " fixed cells." The wandering 

 cells are the polymorphonuclear leucocytes ; the fixed 

 cells are those lining serous cavities and the blood and 

 lymph spaces. When a foreign substance enters the 

 body, it has at first a repellent action on the 

 leucocytes ; later it attracts them in large quantities. 

 Such attraction is called ''positive chemotaxis." In 

 the case of bacteria, chemotactic attraction or repulsion 

 is intimately dependent upon the nature of the micro- 

 organism, and very probably has a definite relation- 

 ship to its virulence. 



We have seen that the invasion of the animal body 

 by a foreign substance is followed by the prompt 

 response on the part of the phagocytic cells. In the 



