LYSIN& 155 



which colors the solution deep red. This change is clearly visible 

 and requires no additional procedure for its demonstration; the 

 liberation of hemoglobin is in itself an indicator of the reaction which 

 has taken place. 



The relation between antigen, immune serum, and complement 

 is quantitative; consequently, if the respective amounts of the three 

 components are correctly proportioned, no free unattached comple- 

 ment will be present in a mixture of them after an appropriate incuba- 

 tion at body temperature is practiced to allow of their union. Usually 

 an hour at 37 C. suffices for this union to take place quantitatively. 



These very important observations of Bordet and Gengou have 

 led to the development of a technic for the diagnosis of infection, and 

 the identification of antigens by the method of complement fixation. 



The underlying principles of the reaction of complement fixation 

 are three: 



(a) The union of specific inactivated immune serum and homologous 

 antigen. 



(b) The quantitative activation of the antigen inactivated specific 

 immune serum complex by non-specific complement; and 



(c) The visible hemolysis that results from the activation of an 

 erythrocyte inactivated specific immune serum complex by non- 

 specific complement. 



The general plan of procedure is to incubate an antigen (as bacterial 

 cells) and inactivated serum and complement in proper proportions 

 for an hour, to permit the three components to unite. A mixture of 

 erythrocytes and specific inactivated hemolytic serum is now added. 

 If the reactive substances are properly proportioned and the inac- 

 tivated serum first added is specific for the antigen (bacteria), no 

 hemolysis will occur when the hemolytic system is added, because 

 all the complement present is bound by the bacteria-immune serum 

 complex. On the contrary, if the inactivated serum is not specific 

 for the bacterial antigen, no union between the two will take place, 

 complement will not be bound, and it is free in the mixture. It will 

 activate the erythrocyte-inactivated immune serum complex, and 

 hemolysis will occur. 



It will be seen that the hemolytic system is added as an indicator; 

 an absence of hemolysis shows a union of bacterial antigen, inactive 

 specific bacterial immune serum and complement. Hemolysis shows 

 that the union has not been formed, the complement was free in the 

 mixture and it united with the hemolytic system, causing hemolysis 



