BACTERIOLYSINS AND H^EMOLYSINS 55 



cholera immunity the close analogy between this and 

 the subject of haemolysis is apparent. Just as, when 

 immunizing an organism against cholera bacilli 

 the organism responds with an increased solvent 

 power for those bacteria, so does the organism 

 respond when it is treated, i.e. immunized, with 

 red cells of another species, by increasing the sol- 

 vent power of its serum for those particular cells. 

 Furthermore, just as the hoemolytic process was 

 seen to depend on the combined action of two sub- 

 stances, one developed in the hasmolytic serum, 

 the other already present in normal serum, so also 

 in the bactericidal process just studied there are 

 two factors. It is easy to understand, therefore, 

 what formerly was not at all clear, why a specific 

 bactericidal serum against cholera, typhoid, or 

 other infectious disease should not act in a test- 

 tube unless there had first been added some normal 

 serum (according to Metchnikoff), or there had 

 been employed a perfectly fresh serum (according 

 to Bordet): simply because in either of these 

 ways the alexin necessary to co-operate with the 

 substance sensibilatrice is introduced. This alexin 

 no longer exists in the immune serum, if this be 

 not perfectly fresh, for we have seen that it decom- 

 poses either on warming, or spontaneously on stand- 

 ing. A bactericidal serum, therefore, that has 

 stood for some time is incapable of dissolving 

 bacteria. It is possible, however, to make an old 



