THEORIES OF IMMUNITY 131 



of substances contained within the plasma or blood serum, which 

 prepare bacteria for phagocytosis to use Wright's terminology has 

 modified somewhat the original conception of phagocytosis as proposed 

 by Metchnikoff. 



The phagocytic theory and the humoral theory of immunity would 

 appear to be in direct opposition. Metchnikoff maintained that the 

 fundamental basis of immunity resides in the phagocytic activity of 

 macro- and microphages. He believed that the humoral immune 

 bodies are derived either from leukocytes or the organs in which 

 they are formed the bone marrow and lymphatic system. The 

 champions of the humoral theory, on the other hand, would attribute 

 the healing principle to soluble substances contained in the body 

 fluids. The leukocytes and other phagocytic cells, according to the 

 extremists who advocate this theory, would be rega/ded as scavengers 

 merely, whose function it is to remove the debris dead bacteria or 

 disabled bacteria after they are overwhelmed by the activity of the 

 soluble natural and immune antibodies. 



A final decision of the importance of cellular and humoral factors 

 in immunity cannot be made at the present time. It is not unlikely 

 that both theories will be modified somewhat as additional evidence 

 accumulates. 



