IMMUNITY REACTIONS 207 



Complement Fixation and the Wassermann Reaction. 



A mixture of antigen and immune substance (e.g., goat serum + 

 goat-rabbit serum) has the property of binding complement. This is 

 recognized by the following: if complement is added to a suspension 

 of red blood corpuscles + amboceptor, hemolysis occurs. If the 

 complement has previously been mixed with antigen + immune 

 substance and we then add the entire mixture to the blood corpuscles 

 + amboceptor, no hemolysis occurs. 



Complement Complement 



+ + 



Amboceptor Amboceptor Amboceptor + Antigen 



+ + + , + 



Erythrocytes Erythrocytes Erythrocytes Immune substance 



Hemolysis No Hemolysia No Hemolysis 



This phenomenon, which is called complement fixation or com- 

 plement deviation, was discovered by O. GENGOU and BORDET. It 

 has acquired great practical significance by its utilization for the 

 recognition of antigen by M. NEISSER and H. SACHS (one billionth 

 c.c. of human blood may be recognized by complement deviation) 

 and also indirectly, through a reaction analogous to the Wassermann 

 reaction for the recognition of luetic infection. 



Admixture of antigen and immune serum may give a precipitate, 

 though only when mixed in definite proportions, otherwise this does 

 not occur. Complement fixation occurs regardless of the occurrence 

 of a precipitation. Since complement is easily adsorbed by many 

 colloids and suspensions, it was natural to suppose that the precipi- 

 tate of antigen and immune substances was the fixing agent. This 

 view is held especially by U. FRIEDEMANN who attributes to eu- 

 globulin the complement binding power of the immune serum. The 

 investigations of DEAN are of great interest to students of colloid 

 chemistry. According to these investigations, much complement 

 is bound when there is a slowly developing turbidity and but little 

 when turbidity develops rapidly. From this aspect a definite develop- 

 ment of surface tension favors binding of the complement. Though 

 it is probable that binding of complement depends on a physical ad- 

 sorption of the visible or invisible precipitate, its mechanism requires 

 further elucidation. The objection that the fixation may occur even 

 without the appearance of a precipitate cannot definitely be proven. 

 We know that albumin particles may aggregate into larger particles 

 without a precipitation, provided the excess of one of the precipitate- 

 forming colloids acts as a protective colloid. On the other hand, it 



