552 IMMUNITY 



phenomena described below (p. 558). Further investigation alone 

 will settle these and various other disputed points, and may 

 remove many of the apparent objections. At present we may 

 say, however, that Ehrlich's theory is the only one which even 

 attempts to explain the cardinal facts of this aspect of immunity. 

 2. The Theory of Phagocytosis. This theory, brought 

 forward by Metchnikoff to explain the facts of natural and 

 acquired immunity, has been of enormous influence in stimu- 

 lating research on the subject. Looking at the subject from the 

 standpoint of the comparative anatomist, he saw that it was 

 a very general property possessed by certain cells throughout 

 the animal kingdom, that they should take up foreign bodies 

 into their interior and in many cases digest and 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 

 " polymorpho-nuclear " finely granular leucocytes of the blood ; 

 and (b) the macrophages, which include the larger hyaline 

 leucocytes, 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 

 rapid 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 organisms are introduced into the tissues of a moderately 

 susceptible animal, 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 showed that when a naturally susceptible 

 animal was immunised, the process was accompanied by the 

 appearance of an active phagocytosis. The ingestion of bacteria 

 by phagocytes is undoubtedly a phenomenon of the greatest 

 importance in the defence of the organism. It is known that 

 amoebae and allied organisms have digestive properties which 

 are specially active towards bacteria, and from what can be 

 directly observed, as well as indirectly inferred, there can be no 

 doubt that such a faculty is also possessed by the phagocytes of 

 the body. Thus bacteria within these cells are in a position 



