BACTERIOLYSIS AND ALLIED PHENOMENA 141 



substance can be rendered active again by contact with the living 

 endothelial cells of the peritoneum. 



The true explanation was given by Bordet, who showed that 

 two substances are necessary for the phenomenon. The one is the 

 inert thermostable substance which occurs in the serum of highly 

 immunized animals, but not in normal animals, and which does 

 not disappear on keeping. The second is a thermolabile sub- 

 stance which occurs in fresh serum, whether from a normal or 

 from an immunized animal. This he showed by demonstrating 

 that, whereas neither heated nor stale immune serum alone, nor 

 fresh serum alone, had the power of leading to the production of 

 Pfeiffer's reaction in vitro, this was caused when the two were 

 used in combination. When a drop of fresh serum from a normal 

 animal, some heated immune serum, and the cholera vibrios are 

 mixed together and incubated, the whole train of phenomena was 

 repeated. 



The thermolabile substance taking part in the process was 

 soon identified as the alexin of the earlier investigators, and the 

 new substance was called by Bordet substance sensibilatrice. His 

 theory was that alexin alone does not unite with the bacteria, 

 unless these have been previously sensitized by the action of the 

 substance in the immune serum. It was shown that the substance 

 sensibilatrice has the power of uniting with the bacteria as follows : 

 A culture exposed to the action of heated immune serum and 

 washed by repeated centrifugalizations with normal saline solution 

 is apparently unaltered, but the bacteria are dissolved by the 

 action of normal serum, which has no action on unsensitized 

 bacteria. Bordet supposed, therefore, that the immune serum 

 alters the constitution of the bacteria in some way, so that the 

 alexin can combine with them subsequently. He compared the 

 process to the opening of a lock, which can only be effected by 

 means of two keys, of which one (the sensibilatrice) must be 

 turned before the other (alexin) can be introduced. 



Bordet found that the essential facts could be reproduced 

 exactly if red blood-corpuscles were substituted for bacteria. It 

 was previously known that the serum of some animals possesses 

 the power of liberating the haemoglobin (haemolysis) from the red 

 corpuscles of other species, just as the serum of some animals 

 can destroy certain bacteria. Bordet showed that the guinea-pig 

 serum has normally no action on the red corpuscles of the rabbit, 

 but that it becomes haemolytic to the latter after a few injections 



