224 GENERAL BIOLOGY OF MICRO-ORGANISMS 



mate estimation of its amount in an unknown mixture. By 

 the use of such a reagent it is possible to show that complement 

 is destroyed or used up in various specific cytolytic, proteolytic, 

 and precipitin reactions. Thus Bordet and Gengou mixed to- 

 gether typhoid bacilh (antigen), heated typhoid-immune serum 

 (amboceptor) and fresh normal serum (complement) and incu- 

 bated the mixture. After an hour the hemolytic amboceptor 

 and sheep's blood cells were added and incubation continued. 

 No hemolysis resulted, showing that the complement added in 

 the first place had been used up, "fixed," as a result of a reaction 

 with the typhoid bacilli and typhoid amboceptor. This is the 

 phenomenon of fixation of complement. Obviously it lends itself 

 to use as a test for the presence of a^ specific antigen or for the pres- 

 ence of specific amboceptor. Its more definite application will 

 require subsequent mention. 



Opsonins.- — Wright and Douglas (1903) showed that blood 

 serum contains a something which affects bacterial cells, soaked 

 in the serum, in such a way that they are more readily ingested 

 by the living leukocytes. To this substance they gave the 

 name "opsonin" {opsono, I prepare victuals for). Substances 

 of this sort are present in normal blood, but are increased as a 

 reaction following infection. It would seem that more than 

 one substance may act upon bacterial cells in this manner, for 

 Neufeld has shown that the opsonic power of normal serum may 

 be destroyed by heating to 56° C, while the similar property of 

 immune serum remains after this treatment. It is not yet con- 

 clusively proven that opsonins are separate substances entirely 

 distinct from bacteriolysins and agglutinins, but it has been shown 

 that opsonic power of a serum does not correspond in its con- 

 centration to that of the other antibodies, and some other element 

 must, therefore, be a factor. Hektoen considers the opsonins 

 to be distinct bodies, different from lysins and agglutinins. 

 The study of opsonins has done much to bring about harmony 

 between the followers of Metchnikoff, with their tendency to 

 emphasize the importance of phagocytosis, and the followers 



