ON IMMUNITY TO BACTERIA 353 



may take place. In the first the immunity appears to depend 

 mainly on bacteriolysis, in the second on phagocytosis. 



In the former case an enormous number of bacilli are killed in 

 the space of a few minutes, very few being recoverable from the 

 peritoneum by washings with normal saline solution, and when 

 this takes place it is found that few or no bacilli make their 

 way into the blood or organs. In such cases the typhoid bacilli 

 which are recovered show marked signs of degeneration, similar, 

 though less marked, to those seen in cholera vibrios in the classical 

 Pfeiffer's reaction ; the staining material becomes collected into 

 the centre of the sheath of the bacillus, which subsequently breaks 

 down into minute granules. This process is usually entirely 

 extracellular, but after a time some of the altered bacilli may be 

 taken up by the phagocytes. It would appear at first sight, there- 

 fore, that in this case the complement -amboceptor mechanism 

 is present in the normal peritoneal fluid, ready to bring about 

 immediate bacteriolysis. This, however, is by no means certain, 

 and it is highly probable that the complement does not exist in 

 this fluid, and that its appearance follows the injection, and is due 

 in some way to the leucopsenia which has been already noted. 

 The nature of this leucopsenia has been much discussed, and is of 

 some importance in regard to this question of the nature of the 

 complement. Metchnikoff and the French school generally 

 maintain that the diminution is due to the actual destruction of 

 the leucocytes phagolysis and that during this destruction or 

 solution of these cells alexin or complement, or, as Metchnikoff 

 calls it, microcytase, is set free. As a matter of fact, there is no 

 evidence that this phagolysis does occur, and more recent researches 

 appear to point to another solution of the leucopaenia. It is true 

 that the fluid withdrawn from the peritoneum by means of a 

 pipette is markedly deficient in cells, but those that are present 

 do not show the marked degenerative changes we should expect 

 if they were undergoing rapid solution. And in any case, it is 

 difficult to believe that an injection of substances such as normal 

 saline, which we know preserves the leucocytes in a lively condition 

 for many hours, should have such a profound destructive effect on 

 them in the peritoneum. Normal saline, it may be pointed out, is 

 capable of producing a most marked leucopaenia when injected 

 into the peritoneal cavity. 



The most probable explanation of the phenomenon is that first 

 suggested by Pierallini, and corroborated by Buxton and Torrey. 



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