COMPLEMEflT-FIXATION TEST 255 



uniting with complement. The indicator is put into the 

 first mixture and the whole is again incubated for at least 

 two hours and examined. If the mixture is clear and color- 

 less wjth a deposit of red corpuscles at the bottom, that would 

 mean that the complement had been bound to the first 

 complex, since it was not free to unite with the second 

 sheep's corpuscles (antigen) — rabbit serum (hemolytic ambo- 

 ceptor) complex^ — and destroy the corpuscles. Hence if the 

 complement is bound in the first instance, the specific ambo- 

 ceptor for the first antigen must have been present in the 

 blood, that is, the animal was infected with the organism in 

 question. Such a reaction is called a "positive" test. 



On the other hand, if the final solution is clear but of a red 

 color, that would mean that complement must have united 

 with the corpuscles^hemolytic amboceptor system — and 

 destroyed the corpuscles in order to cause the clear red 

 solution of hemoglobin. If complement united with this 

 system it could not have united with the first system, hence 

 there was no specific amboceptor there to bind it; no specific 

 amboceptor in the animal's blood, means no infection. 

 Hence a red solution is a "negative test." 



In practice all the different ingredients must be accurately 

 tested, standardized and used in exact quantities, and a test 

 must also be run as a control with a known normal blood 

 of an animal of the same species as the one examined. 



The complement-fixation test might be applied to the 

 determination of unknown bacteria, using the unknown cul- 

 ture 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 medicolegal cases in exactly the above outlined 

 manner and is very delicate and accurate. 



