PHAGOCYTOSIS 209 



Adaptability seems to be one of the innate properties 

 of protoplasm, and immunity is but an instance of adapta- 

 bility. It might be expected, therefore, that immunity 

 towards infection will become established, more or less 

 completely, when the need for it arises ; and we find that 

 this is the case, however difficult it may be to explain the 

 mechanism by which it is attained. 



The Role of the Serum in Phagocytosis 



The fact that in an immunised animal, no sooner does 

 the virulent organism gain access than the leucocytes 

 migrate to the site of infection, surround the invaders, 

 ingest and so destroy them, was at one time ascribed by 

 Metchnikoff to " education," i.e. modification, of the 

 leucocytes ; but since the serum of the immunised animal 

 injected into a non- immunised one causes the leucocytes in 

 the latter to behave in the same manner as they do in the 

 immunised animal, the effect must be due to something 

 in the plasma or serum, and Metchnikoff ascribed the 

 action to substances, " stimulins," which heighten the 

 activity of the leucocytes. Later work has not confirmed 

 this view, and no certain proof of the existence of stimulins 

 is forthcoming, although Leishman attributed a stimulin 

 action to thermostable substances in the serum in typhoid 

 and Malta fevers. Subsequently Metchnikoff conceived 

 the serum as acting, not on the leucocytes, but on the 

 microbe, causing it to become positively chemotactic and 

 no longer to repel, but to attract the phagocytes. Con- 

 siderable support was given to this view by the work of 

 Wright and Douglas, who, by a modification of Leishman's 

 ingenious method for quantitatively estimating phago- 

 cytosis, emphasised the importance of the serum in the 

 mechanism of phagocytosis. 



Neufeld and Eimpau also concluded that substances, 



