70 STUDIES IX IMMUNITY. 



the serum of immunized animals. It may be that this specific pre- 

 ventive substance has of itself some harmful effect on the vibrios 

 that predisposes them to feel the power of the bactericidal sub- 

 stance. At least the preventive substance alone has no real anti- 

 septic properties, for it cannot kill a culture or even pre vent its 

 growth. It lends to the bactericidal substance present with it, 

 however, a character of specificity. When fresh, the serum of 

 vaccinated animals has both substa?ices, but when heated to 55 degrees 

 for a few moments or kept for a long time, the preventive substance 

 alone remains; the addition of fresh serum is necessary to restore 

 to it its bactericidal property. It is indifferent whether this fresh 

 serum containing the bactericidal substance comes from a normal 

 animal or an immunized animal, or even from an animal that has 

 just died of such an infection as that caused by anthrax or the 

 pneumococcus. 



Fifth. Is the bactericidal substance present in equal amounts in 

 normal and in immunized animals ? While the immunized animal 

 is forming the specific preventive substance, does it increase to any 

 extent the bactericidal substance present before immunization? 

 One may determine at least in an approximate manner the amount 

 of bactericidal substance present in normal and immune serum. 

 Let us mix a loop of well-immunized guinea-pig serum with a drop 

 of culture. We have established, let us suppose, by a preliminary 

 experiment, that a drop of this mixture placed with a drop of normal 

 serum gives a complete Pfeiffer's phenomenon ; that is to say, in a 

 drop of this emulsion there is enough preventive substance present. 

 It must be noted that the fresh preventive substance alone may 

 give metamorphosis of the vibrio. But under the conditions noted, 

 that is, one loop of the serum to one drop of cholera emulsion, 

 no distinct phenomenon takes place. But, as we have already 

 shown, there is enough preventive substance there ; it is the bacteri- 

 cidal substance that is lacking. If we add another loop of the same 

 serum to the mixture, the phenomenon occurs very distinctly and 

 becomes still more complete if a third loop is added. At the same 

 time, if we add normal serum to a similar mixture of cholera emul- 

 sion and active preventive goat serum, we find that only three 

 loops are necessary to bring about the usual degree of the phe- 

 nomenon. The amount of fresh serum, whether from a normal or 



