ANTIGENS, HAPTENS, ANTIBODIES, ETC. 437 



In complement fixation reactions almost all the C2 

 and C4, 75 per cent, of the CI and 25 per cent, of the C3 

 are removed. Elimination of C3 or C4 does not influence 

 the fixation of the other components, but inactivation 

 of CI and C2 by heat inhibits the fixation of C3 and C4. 

 The inactivation of complement by fixation is due mainly 

 to the removal of CI, C2 and C4 from the serum. In the 

 absence of Cl, components C2 and C4 are not fixed, 

 but Cl is fixed in the absence of all the other components. 

 Human complement has been fractionated in the 

 same way and shown to contain components Cl, C2, 

 C3 and C4 which afe almost, but not quite, identical 

 with the corresponding fractions of guinea-pig com- 

 plement. 



The present state of our knowledge of the structure of 

 complement can be summarised as follows (illustrated 

 diagrammatically by Fig. 10) : — 



End-piece. — Albumin and pseudo globulin ; heat 

 labile ; carries 70 to 100 per cent, of C4 and 20 to 

 40 per cent, of C3 ; constitutes 0-2 per cent, of 

 total serum protein ; contains about 10 per cent, 

 of carbohydrate ; very little adsorbed in com- 

 plement fixation reactions, but necessary for lytic 

 reactions. 



Encl-picc( 



Mid-piece 



Heat la})ile 



n 



Fourth Comjionent 



Third Component 



Heat stable 



Fig. 10 



