496 IMMUNITY 



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 

 favourable to their destruction and do in many instances 

 become destroyed. In fact, observations on phagocytosis in 

 vitro show that such destruction may in the case of some 

 organisms occur so rapidly that the actual number observable in 

 the leucocytes is no indication of the activity of the process. 

 In other instances, e.g. in gonorrhoea, the ingested organisms 

 would appear to survive a considerable time without undergoing 

 change. Undoubtedly phagocytosis is of the highest importance 

 in active immunity, as by its means organisms which would not 

 undergo an extra-cellular death may be killed off. In the process 

 of immunisation of a susceptible animal we see a negative or 

 neutral chemiotaxis becoming replaced by positive chemiotaxis. 

 This has bee a explained by Metchnikoff as due to an education 

 or stimulation of the phagocytes. The recent work on opsonins 

 shows, however, that this is not the case, as leucocytes from an 

 immunised animal are not more active in this direction than 

 those of a normal animal, the all-important factor being the 

 development of an opsonin in the immune animal. Thus this 

 phase of immunity comes to be merely a part of the subject of 

 anti-substances in general. 



The digestive ferments of phagocytes or cytases are, according 

 to Metchnikoff, retained within the cells under normal conditions, 

 but are set free when these cells are injured, for example, when 

 the blood is shed. They then become free in the serum by 



