74 IMMUNE SERA 



first species acquires an uncommonly increased 

 power to dissolve and to agglutinate the erythro- 

 cytes of the second species. This increased haemo- 

 lytic power shows itself not only in vivo, so that an 

 animal so treated is able to cause red cells injected 

 into it to rapidly dissolve and disappear, but it 

 shows itself also in vitro when the serum of this 

 animal is used. The process consists in the com- 

 bined action of two substances, that which is excited 

 in response to the injection, the substance sensi- 

 bilatrice, and the alexin of normal serum. 



Analogy between the Bacteriolytic and Haemolytic 

 Processes. If we now recall the main points in 

 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 haemolytic process was 

 seen to depend on the combined action of two sub- 

 stances, one developed in the haemolytic 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 



