450 IMMUNITY. 



kingdom that they should take up foreign bodies into their 

 interior and in many cases destroy them. On extending 

 his observations to what occurred in disease, he came to 

 the conclusion that the successful resistance of an animal 

 against bacteria depended on the activity of certain cells 

 called phagocytes. In the human subject he distinguished 

 two chief varieties, namely (a) the microphages, which are 

 the multinucleated leucocytes of the blood, or more correctly, 

 those with multipartite nucleus ; and (<) the macrophages, 

 which include the larger uninucleated leucocytes, endothelial 

 cells, connective tissue corpuscles, and, in short, any of the 

 larger cells which have the power of ingesting bacteria. In- 

 susceptibility 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 leuco- 

 cytes. 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 



