254 RECEPTORS OF THE THIRD ORDER 



and on the further fact that if the combination with one 

 amboceptor-antigen system is once formed, it does not dis- 

 sociate so as to Hberate the complement for union with the 

 second amboceptor-antigen system. If an animal is infected- 

 with a microorganism and a part of its defensive action con- 

 sists in destroying the organisms in its blood or lymph, then 

 it follows from the above discussion of cytolysins that there 

 will be developed in the blood of the animal amboceptor 

 specific for the organism in question. If the presence of this 

 specific amboceptor can be detected, the conclusion is war- 

 ranted that the organism for which it is specific is the cause 

 of the disease. Consequently what is sought in the- " com- 

 plement-fixation test" is a specific amboceptor. In carrying 

 out the test blood serum from the suspected animal is col- 

 lected, heated at 56° for half an hour to destroy any comple- 

 ment it contains and mixed in definite proportions with the 

 specific antigen and with complement. The antigen is an 

 extract of a diseased organ (syphilitic fetal liver, in syphilis), 

 a suspension of the known bacteria, or an extract of these 

 bacteria. Complement is usually derived from a guinea- 

 pig, since the serum of this animal is higher in complement 

 than that of most animals. The blood of the gray rat con- 

 tains practically as much. If the specific amboceptor is 

 present, that is, if the animal is infected with the suspected 

 disease, the complement will unite with the antigen-ambo- 

 ceptor system and be "fixed," that is, be no longer capable 

 of uniting with any other amboceptor-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 physicochemical 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 incubator for 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 ambo- 

 ceptor specific for the corpuscles, which is capable also of 



