ON IMMUNITY TO BACTERIA 357 



process of recovery from an infective process. The mechanisms 

 themselves are now fairly well known, the nature of the substances 

 concerned therein thoroughly investigated, and their sources ascer- 

 tained with some degree of probability; but when we come to 

 apply our knowledge to the actual events of the living body, our 

 difficulties begin in earnest. It is in this field of research that 

 future advances are most to be expected. 



In conclusion, let us emphasize the enormous importance which 

 we have been led to attach to the leucocytes in the struggle against 

 the infective diseases. In all probability the substance we call 

 alexin, thermolabile opsonin, or complement, and which, in one 

 or other of its actions, constitutes the first line of defence, is 

 formed by the polynuclear leucocytes. In itself its action is but 

 slight, and it requires to be supplemented either by the leucocyte 

 itself, in case the reaction is mainly phagocytic, or by the action 

 of amboceptor, which we believe to be derived from the masses of 

 leucocytes known as lymphoid tissue. Again, the bacterium may 

 give off toxin, especially if it has escaped being taken up by the 

 leucocyte, and in this case these cells exhibit another phase of 

 their protean activities, for that they can actually absorb toxin in 

 solution appears quite certain. If this action fails the last resource 

 is the neutralization of the toxin by antitoxin ; and although this 

 cannot be regarded as definitely proved, there is at least some 

 reason to believe that this may be formed in the lymphoid tissues. 

 Lastly, certain facts would seem to show that it is quite likely 

 that even the compound of toxin and antitoxin is by no means 

 inert unless it has been taken up by the leucocytes, and that the 

 action of the latter substance is simply to prepare the former so 

 that these cells may attack it. In every phase of the struggle 

 against the bacteria the leucocyte appears, on certain or presump- 

 tive evidence, as the particular cell to which the defence of the 

 body is entrusted. 



