R. L. KAHN 855 



a definite precipitate. If the readings in the three tubes are: — in the first, ++ in 

 the second, and + + + + in the third, then the final result is + + . The precipitates 

 in the quantitative procedures are read on a positive and negative scale. A precipi- 

 tate which is either + + + + , + + + , or ++ is read "positive," while all others 

 are read "negative." The final result expressed in reacting units is obtained by 

 multiplying by 4 the highest serum dilution giving a positive reaction. 



SPECIAL ANTIGEN 



Standard antigen necessarily had to be of a conservative degree of sensitivity. 

 In isolated cases it is desirable to employ a more sensitive antigen than the standard. 

 Since the amount of ether extractives in an antigen is directly related to its sensitivity, 

 a special antigen more sensitive than standard antigen was produced containing a 

 somewhat larger amount of ether extractives than the standard. Thus, in the prepara- 

 tion of special antigen from 25 gm. of powdered beef heart, the ether extractions are 

 limited to five minutes, employing 50, 50, 40, and 40 cc. of ether, respectively. In 

 other respects it is prepared like standard antigen. This antigen is employed in the 

 presumptive procedure and may be employed in special cases in other procedures. 

 The sensitivity of this antigen is also standardized to a uniform degree. 



RELATION BETWEEN PRECIPITATION AND COMPLEMENT FIXATION' 



There is little doubt but that the precipitation and complement fixation reactions 

 detect the same substance or substances in syphilitic serum. This was suggested by 

 Jacobstahl in 1908. In my laboratory the identity of precipitin and complement- 

 fixing substances was studied by adding complement to completed Kahn tests, and 

 after incubation, following with amboceptor and washed corpuscles. The results of 

 1,800 such examinations showed 93 per cent absolute agreement, 5 per cent relative 

 agreement, and 2 per cent disagreement. With better understanding of precipitation 

 and complement fixation, closer agreement might be obtained. 



The method employed consisted in adding o.i cc. (2 units) of complement to 

 0.2 cc. of the serum-antigen suspension mixture of the third tube of the Kahn test 

 which contains the least amount (0.0125 cc.) of the suspension. This was incubated 

 in the water bath at 37° C. for thirty minutes after which o.i cc. of antisheep-ambo- 

 ceptor (2 units) solution and 0.1 cc. of a 5 per cent sheep-cell suspension were added 

 and incubated about fifteen minutes, until the serum and antigen control tubes 

 showed complete hemolysis. Complete fixation of complement was read + + + + ; 

 partial fixation, + + + , + + , +, and +, respectively. 



The completed Kahn test may be utilized with any complement fixation tech- 

 nique. It is well to use the tube of the Kahn test which contains the least amount of 

 antigen suspension. From this tube an amount of serum-suspension mixture is 

 pipetted off, corresponding to the amount of serum employed in the desired comple- 

 ment fixation technique, complement added, incubated, and the complement fixation 

 test completed in the usual manner. This complement-fixation-Kahn-precipitation 

 method is of special value in presenting this subject to students. It also offers a 



' Kahn, R. L., Landau, J. L., and McDermott, E.: Proc. Soc. Exper. Biol. £r Med., 24, 775. 1927 



