THE TECHNIQUE OF SERUM REACTIONS 325 



it on a number of successive days with 0.5 c.c. of 5 per cent cells and 

 0.5 c.c. of 10 per cent guinea-pig sera, readings being made at the end 

 of a half hour. When complement is then subsequently titrated with 

 2 such amboceptor units and 0.5 c.c. of 5 per cent red cells, it is usually 

 found that the minimal hemolytic dose of complement lies between 0.2 

 and 0.25 c.c., and in the actual tests twice this minimal hemolytic dose 

 of complement is used with the 2 units of amboceptor. The daily 

 titration of complement frequently shows marked variations even 

 though the complement is obtained from a number of guinea-pigs. By 

 always using twice the amount which just lakes with 2 units of ambo- 

 ceptor, one keeps the actual amount of complement as nearly as pos- 

 sible constant. 



Obviously one can titrate the strength of the reaction by varying 

 the amount of any one of the three ingredients which primarily enter 

 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 tubes 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 expresses 

 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 ................... ........................ -f- ~h 



If the main tube shows incomplete inhibition and the second tube 



none or little ............................. ................ + 



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 



principles underlying the preceding tests for the determination of 

 suspected 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 

 temperature. At the end of an hour the mixture was tested for free 



