COMPLEMENT-FIXATION TEST 275 



unite with the antigen-amboceptor system and be "fixed," 

 that is, be no longer capable of uniting with any other 

 ambocep tor-antigen system. No chemical or physical means 

 of telling whether this union has occurred or not, except as 

 given below, has been discovered as yet, though doubtless 

 will be by physico-chemical tests, nor can the combination 

 be seen. Hence an "indicator," as is so frequently used in 

 chemistry, is put into the mixture of antigen-amboceptor- 

 complement after it has been allowed to stand in the incu- 

 bator for one-half to one hour to permit the union to become 

 complete. The "indicator" used is a mixture of sheep's 

 corpuscles and the heated ("inactivated") blood serum of a 

 rabbit which has been injected with sheep's blood corpuscles 

 and therefore contains a hemolytic amboceptor specific for 

 the corpuscles which is capable also of uniting with 

 complement. The indicator is put into the first mixture 

 and the whole is again incubated and examined. If the 

 mixture is clear and colorless with 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 amboceptor) complex— and destroy the 

 corpuscles. Hence if the complement is bound in the first 

 instance, the specific amboceptor 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." 



The scheme for the test may be outlined as follows: 



Antigen + Patient's serum, heated + Complement 



(specific for the (unknown amboceptor) (derived from 



amboceptor sought) guinea-pig's serum) 



