REACTION OF THE HOST TO INFECTION 217 



reagent which serves for the detection of complement and the 

 approximate 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 

 together typhoid bacilli (antigen), heated typhoid-immune 

 serum (amboceptor) and fresh normal serum (complement) 

 and incubated the mixture. After an hour the hemolytic ambo- 

 ceptor and sheep's blood cells were added and incubation con- 

 tinued. 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 presence 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 entirety 

 distinct from bacteriolysins and agglutinins, but it has been shown 

 that opsonic power of a serum does not correspond to 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 agglutin- 

 ins. The study of opsonins has done much to bring about 

 harmony between the followers of Metchnikoff, with their tendency 



