THE TECHNIQUE OF SERUM REACTIONS 271 



into it the antigen, the patient's serum, or the complement, and all 

 three of these systems have been proposed and used with success by 

 different workers. The method which is most commonly used is that 

 of- Citron and consists of using diminishing quantities of both antigen 

 and patient's serum. Citron in addition to the main tube uses one 

 additional tube containing one-half the amount of patient's serum 

 and one-half the amount of antigen used in the main tube, and he ex- 

 presses his results as follows: 



If both tubes show complete inhibition 



If the main tube shows complete inhibition and the half dose 



tube almost complete ............ . ..................... + + + 



If the main tube shows complete inhibition and the second 



tube faint inhibition ..................................... + + 



If the main tube shows incomplete inhibition and the second 



tube none or little . .................................... -f 



If the main tube shows very faint inhibition and the second 



tube shows none .............................. ......... 



" + + ++," " + ++," and " + + " are regarded as conclusive results; 

 " + " as a probable positive; " " as suspicious merely. 



The Determination of Antigen by Complement Fixation. The prin- 

 ciples underlying the preceding tests for the determination of sus- 

 pected antibodies may be equally applied to the determination of 

 suspected antigen. In the former case it was necessary to bring the 

 serum to be tested into contact with the antigen specific for the suspected 

 antibody, in the presence of complement, and at a suitable tempera- 

 ture. At the end of an hour the mixture was tested for free comple- 

 ment by the addition of hemolytic amboceptor and red blood cells. 

 In testing for antigen, the procedure is reversed, in that the serum or 

 other substance (bacterial extract) to be tested is brought into contact 

 with an antibody specific for the antigen, in the presence of complement ; 

 and at the end of an hour at suitable temperature, free complement 

 is again determined by hemolytic reaction as before. 



When dealing with bacterial antigen, it is necessary, therefore, to 

 prepare a highly potent immune serum against the bacteria which 

 contain the specific antigen which is sought. 



Thus in testing for typhoid-bacillus antigen in the serum of a patient, 

 the substances required are as follows: 



1. Complement: obtained from fresh guinea-pig serum. It is best 

 to titrate the complement when possible, using for the test double the 



