274 RECEPTORS OF THE THIRD ORDER 



recognition of small amounts of specific protein that is in use 

 today. 



This method is usually spoken of as the "complement- 

 fixation test" ("Wassermann test" in syphilis) and is appli- 

 cable in a great variety of microbial diseases; but it is of 

 practical importance in those diseases only where other 

 methods are uncertain— syphilis in man, concealed glanders 

 in horses, contagious abortion in cattle, etc. A better name 

 would be the ''Unknown Amboceptor Test," since it is the 

 amboceptor that is searched for in the test by making use 

 of its power to "'Rx^' complement. 



The principle is the same in all cases. The method 

 depends, as indicated above, on the ability of complement 

 to combine with at least two amboceptor-antigen systems, 

 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 liberate 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 

 consists 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 warranted that the organism for which it is 

 specific is the cause of the disease. Consequently what is 

 sought in the "complement-fixation test" is a specific ambo- 

 ceptor. In carrying out the test, blood serum from the sus- 

 pected animal is collected, heated at 56° for half an hour to 

 destroy any complement 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 contains practically as much. If the 

 specific amboceptor is present, that is, if the animal is 

 infected with the suspected disease, the complement will 



