46 STUDIES IN IMMUNITY. 



substance in normal animals and the specific bactericidal substance 

 in immunized animals. They are both destroyed at about 60 C., 

 and sera heated to this temperature become excellent culture 

 media. In both instances the power is lodged within the phag- 

 ocytes. But these two substances may be sharply distinguished 

 from one another by the fact that the less active substance of nor- 

 mal serum is in no sense specific and attacks vibrios indifferently, 

 while the other, in vaccinated animals, is very powerful and is 

 highly specific. The normal bactericidal substance is harmful for 

 attenuated vibrios, whatever may be their origin, but the specific 

 bactericidal substance is fatal only for a definite race of vibrios. 

 The two substances would appear, then, to be quite different. 



What is more, the bactericidal substance in the serum of immu- 

 nized animals differs from the preventive substance in the same 

 serum. The researches of C. Frankel and Sobernheim * have shown 

 that the preventive substance resists a prolonged heating to 70 

 degrees, whereas the bactericidal substance is entirely destroyed at 

 this temperature. Serum heated to 70 degrees is quite as capable as 

 fresh serum of conferring passive immunity; in other words, the pre- 

 ventive substance is quite unaltered by this temperature. It may 

 be noted that the essential property of this preventive substance 

 lies in causing an intense bactericidal property in the treated 

 animals, as was well established by Frankel and Sobernheim in 

 their important study. These experimenters injected serum from 

 animals vaccinated against cholera into guinea-pigs, and noted on 

 the following day that the serum of these guinea-pigs had become 

 energetically bactericidal for Koch's vibrio. The same result was 

 observed in animals given injections of serum heated to 70 degrees 

 and thereby deprived of its own bactericidal property. For the mo- 

 ment, the fact of importance to us is, that the preventive substance 

 which immunizes animals differs from the bactericidal substance. 



It would seem, then, that we must consider at least three import- 

 ant active substances in immunity : two different bactericidal sub- 

 stances, and a preventive substance. But, as a matter of fact, the 

 matter is not so complicated. It would seem that the bactericidal 

 substances of normal and of immunized animals which differ in the 

 matter of specificity are, in reality, quite identical. The feeble bac- 



* Fraenkel and Sobernheim, Hygienische Rundschan, Nos. 3 et 4, 1894. 



