210 STUDIES IN IMMUNITY. 



has a slight and non-specific bactericidal activity affecting, generally, 

 only attenuated bacteria. But in presence of the preventive sub- 

 stance of an immune serum the alexin energetically attacks that 

 particular race of bacteria for which the immune serum is specific. 

 The activity of the alexin against other bacteria is not, however, 

 increased. 



In short, the preventive substance, in itself not bactericidal, acts 

 specifically on a cell by increasing the destructive effect of the 

 alexin on it. We now call the preventive substance the " sensitizer " 

 as it sensitizes the cell to the alexin. 



It may be stated at this point that the specific hemolytic sera, 

 first demonstrated by us in 1898, agree in general characteristics 

 with the bacteriolytic sera, and any conception of one generally 

 applies to the other. The majority of observers and particularly 

 Ehrlich and Morgenroth agree with us on this point. The only 

 difference lies in the cell affected, and hemolysis corresponds to 

 bacteriolysis. 



B. When an immune serum, as, for example, cholera serum, is 

 given to a normal animal, the introduction of the preventive sub- 

 stance (sensitizer) transmits the specific bactericidal power to this 

 animal's body fluids. The introduction of an alexin, present, of 

 course, in fresh immune serum, is not necessary and therefore a 

 heated immune serum will serve equally well. As we stated in 1895, 

 "It is quite evident why the presence of the bactericidal sub- 

 stance (alexin) is not indispensable for the preventive activity of 

 serum. It is not characteristic of this serum alone, but is present 

 in normal serum as well, and the animal injected with cholera serum 

 already possesses this substance, so that its addition is unneces- 

 sary. What the normal animal does not possess is the preventive 

 substance, and it is this substance, then, that must be given." As 

 soon as the animal obtains the preventive substance it has the two 

 factors necessary for intense specific bacteriolysis. On bleeding 

 such an animal we find that its serum contains the preventive sub- 

 stance as well as the normally present alexin and will, therefore, 

 destroy the vibrio. "The encounter of the two substances pro- 

 duces as energetic a bactericidal power in the animal body as in a 

 test tube." 



As we have already stated, this theory is also applicable to hemo- 



