ON THE MODE OF ACTION OF PREVENTIVE SERA. 89 



heated immune serum contains only the preventive substance, its 

 strong antiseptic power against the vibrio may be restored by 

 mixing with it fresh normal serum, which in itself is only faintly 

 bactericidal. This simple experiment led us to the rather para- 

 doxical conclusion that' " two sera, neither of which is distinctly 

 bactericidal, form a mixture which has marked antiseptic proper- 

 ties against vibrios." 



Only a very small amount of preventive substance is necessary to 

 endow the preformed substance of normal serum with great activity 

 and with specificity. Normal serum heated to 55 degrees loses its 

 power to form a bactericidal mixture with preventive serum. 

 Edema fluid, aqueous humor, etc., from a normal guinea-pig differ 

 from normal serum, since, mixed with preventive serum and an 

 emulsion of vibrios, they cause no metamorphosis. The preventive 

 substance in immunized animals passes more rapidly into the edema 

 fluid than does the bactericidal substance as is shown by the fact 

 that the edema fluid of a vaccinated guinea-pig, which does not 

 cause granular transformation, will produce it on the addition of a 

 small amount of fresh serum. 



We were led to the conclusion that the intense bactericidal prop- 

 erty of immune serum is due to the combination of two substances ; 

 one, the specific preventive substance which resists heating to 60 

 degrees or even more; the other, the bactericidal substance properly 

 speaking, which is not specific and in itself is only slightly active and 

 is present in normal as well as in immunized animals. This latter 

 substance is sharply differentiated by the fact that it is destroyed 

 by keeping for any length of time, or by heating to 55 degrees. The 

 serum of vaccinated animals when fresh contains both substances ; it 

 may easily be deprived of one of them by heat, but the original prop- 

 erties of the serum may be restored synthetically by adding to it 

 the part that has been lost, that is to say, the bactericidal substance 

 (or alexin) of normal serum. 



Since experiment has shown us that the simultaneous presence of 

 both substances is necessary to constitute bactericidal power against 

 the vibrio we must now consider what the respective action of each 

 substance in the mixture is. If we add to an emulsion of vibrios 

 preventive serum alone, that is to say, serum previously heated for 

 half an hour to 60 degrees, we find that the organisms remain 



