PFEIFFER'S PHENOMENON. 249 



of contact with living tissue cells, supposedly the 

 endothelial cells of the peritoneum. According to 

 this conclusion, an inactive serum could become 

 active again only after its introduction into the 

 body. 



It remained for Bordet to show, on the con- 



., ..,,.. ,, and React i A n- 



trary, that contact of the serum with living cells tion. 

 is not necessary to render it active for bacterici- 

 dal experiments in vitro. It is sufficient to add 

 to the heated immune serum a small amount of 

 fresh normal serum from some normal animal, 

 the quantity of normal serum which is used not 

 being in itself bactericidal. Under these condi- 

 tions, then, we have to do with two serums which, 

 when combined, are bactericidal, but when sepa- 

 rated are inactive. The destruction of the active 

 property of a serum by heat or by other means is 

 called inactivation, and the re-establishment of its 

 power by the addition of fresh normal serum is 

 reactivation. The immune serum, when heated to 

 55 to 60 C., loses something which is essential to 

 its activity, and this something may be replaced 

 by the normal serum. That the substance in the 

 normal serum is identical with that which was de- 

 stroyed in the immune serum is indicated by the 

 fact that it is destroyed by the same degree of 

 heat; a heated normal serum will not reactivate 

 an immune serum. 



The conclusion of Bordet that the bactericidal TWO sub- 

 power of a serum depends on the combined action Bactericida? 

 of two substances has been substantiated by numer- s 

 ous investigators. These are the substances which 

 in recent years have become familiar under the 

 names of amboceptor and complement and their 

 various synonyms (see p. 256). One of them, the 



