ON IMMUNITY TO BACTERIA 341 



out by Hoke, Muir, and others. Now, if this process goes on in 

 the living body, it can only indicate that complement (and thermo- 

 labile opsonin) can never occur in a free state in the organs, and 

 hence that bacteriolysis does not occur in these regions. It must 

 be pointed out, however, that amboceptor (or thermostable opsonin) 

 is not absorbed in this way, so that there would be but little inter- 

 ference with phagocytosis. It is quite possible that this absorp- 

 tion of complement does take place in order to avoid the liberation 

 of endotoxin consequent on bacteriolysis. 



The mobilization of the patient's defences is usually manifested 

 by an increase in the number of the leucocytes in the circulating 

 blood. This subserves two functions : there is a greater number 

 of leucocytes to act as phagocytes and there is an increase in the 

 possible number of cells which may produce alexin or complement. 

 Hence we find leucocytosis present almost invariably in cases in 

 which bacteria gain access to the blood ; the exceptions being, 

 firstly, very severe infections in which the defensive reaction fails, 

 and, secondly, a group of diseases of which typhoid fever is the 

 best example. The meaning of the first exception is tolerably 

 clear, that of the second not at all apparent. 



The mechanism by which this increased output of leucocytes is 

 produced is chemotaxis. The bacteria in the blood-stream produce 

 a toxin which, in comparatively mild infections or in severe 

 infections in an animal possessed of strong natural immunity, 

 attacks the leucocytes. These cells will, therefore, tend to leave 

 the region in which they are formed and in which they are lying 

 dormant and make their way into the blood, the amount of toxin 

 being greater in the latter than the former situation. At the same 

 time, as Muir has so conclusively shown, there is an increase in 

 the functional activity of the bone-marrow, which manifests itself 

 in an increase of the leucocyte (and especially polynuclear 

 leucocyte) producing cells. A double process goes on, leucocytes 

 being formed more rapidly and attracted from the bone-morrow 

 as soon as they are produced. That this is due to chemotaxis is 

 shown by the fact that it follows the injection of bacterial toxins 

 and other products into the blood-stream, if not too virulent or in 

 too large amount. 



When either of these conditions occurs we see the opposite 

 result a leucopaenia, or at least an absence of leucocytosis, and 

 this is always an extremely bad omen when it occurs in those 

 infectious processes where leucocytosis ordinarily takes place. 



