STIMULINS 255 



cytes, whereby they are attracted, whereas formerly 

 repelled, by the products of bacterial development, or to 

 substances which stimulate the action of the leucocytes. 

 The germicidal, inhibitory, and bacteriolytic actions of 

 the body-fluids may also be enhanced. It seems probable 

 also in certain instances that the side-chains or receptors 

 having an affinity for the toxin become in some way 

 destroyed or used up, so that further fixation of the 

 particular toxin cannot take place. 



It is to be noted, as Metchnikoff has pointed out, that 

 immunity is much more rapidly acquired against micro- 

 organisms than against their toxins. In nature, it is 

 principally against micro-organisms that the body requires 

 protection. 



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, when virulent 

 organisms gain access, the leucocytes migrate to the site 

 of infection, surround the invaders, ingest and so destroy 

 them, was at one time ascribed by Metchnikoff simply 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 sub- 

 stances, " stimulins," which heighten the activity of the 

 leucocytes. Later work has not confirmed this view, and 



