490 IMMUNITY. 



endothelial cells, connective tissue corpuscles, and, in short, 

 any of the larger cells which have the power of ingesting 

 bacteria. Insusceptibility to a given disease is indicated 

 by a great activity on the part of the phagocytes, different 

 varieties being concerned in different cases, an activity 

 which may rapidly destroy the bacteria and prevent even 

 local damage. If the animal is moderately susceptible 

 and the organisms are introduced into the subcutaneous 

 tissue, there occurs an inflammatory reaction with local 

 leucocytosis, which results in the intracellular destruction 

 of the invading organisms. Phagocytosis is regarded by 

 Metchnikoff as the essence of inflammation. He also 

 showed that the bacteria may be in a living and active 

 state when they are ingested by leucocytes. On the other 

 hand, he found that in a susceptible animal phagocytosis 

 did not occur or was only imperfect. He also found that 

 when a naturally susceptible animal was immunised, the 

 process was accompanied by the appearance of an active 

 phagocytosis. The leucocytes and other cells are guided 

 in their attack on the organisms by chemiotaxis, a process 

 which has already been explained. According to this 

 theory, in the process of immunisation by attenuated 

 cultures the phagocytes are so educated by dealing with 

 the bacteria in an attenuated condition that they can 

 ultimately destroy them even in a highly virulent state. 



The work of Metchnikoff has been of great importance 

 in demonstrating one of the chief means possessed by the 

 body of dealing with invading organisms, and for a time 

 his theory obtained considerable support as an explanation 

 of the facts of immunity. The insufficiency of the theory, 

 however, was at once apparent when the method of im- 

 munising against a toxine was discovered; and the facts 

 discovered later, with regard to the action of antimicrobic 

 sera, showed that the cellular ingestion of bacteria was not 

 the most important factor in immunity against the living 

 organisms. The theory as originally propounded is, ac- 

 cordingly, no longer tenable, and even if it were consistent 

 with facts it only removes the property of immunity a step 



