COMPLEMENT-FIXATION TEST 277 



original ingredients. This latter substance is called an 

 indicator. Without discussing the theory of indicators, 

 it will be sufficient to point out that they are usually 

 substances which will show a visible change in the presence 

 of an excess of one of the reagents. In this case the excess, 

 if any, will be complement. The indicator must react with 

 complement. Therefore it must contain antigen and its 

 specific amboceptor. The antigen and amboceptor which 

 will actually show a change when antigen is acted on by 

 complement are red blood corpuscles for antigen and ambo- 

 ceptor contained in the serum of an animal immunized against 

 these same red corpuscles. Red corpuscles in suspension 

 in physiological normal salt solution give a reddish, opaque 

 fluid. If the red corpuscles are destroyed by complement, 

 the hemoglobin will go into solution and the reddish, opaque 

 fluid will change to a clear, transparent, red colored fluid. 

 If corpuscles are not destroyed, they will in a few hours 

 settle to the bottom of the tube as a reddish deposit. 



Complement can not be in excess if there is amboceptor 

 in the original mixture. The indicator will show no change. 

 The unknown amboceptor has been found to be present. 

 Hence the test is positive. If the corpuscles are destroyed 

 a red solution will result and complement must have been 

 free to unite with the second amboceptor. If free, it did 

 not unite with the amboceptor in the first mixture because 

 there tvas none present. No amboceptor, no disease. Hence 

 a clear red solution is a negative test. 



The complement-fixation test might be applied to the 

 determination of unknown bacteria, using the unknown 

 culture as antigen and trying it with the sera of different 

 animals immunized against a variety of organisms, some 

 one of which might prove to furnish specific amboceptor for 

 the unknown organism and hence indicate what it is. The 

 test used in this way has not been shown to be a practical 

 necessity and hence is rarely employed. It has been used, 

 however, to detect traces of unknown proteins, particularly 

 blood-serum proteins, in medico-legal cases in exactly the 

 above outlined manner, and is very delicate and accurate. 



