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INFECTION AND RESISTANCE 



cholesterin increase the delicacy of the reaction in that more cases 

 react positively with such an antigen than with the uncholesterinized 

 preparations. The experience of Hopkins and Zimmermann, how- 

 ever, would indicate that great caution must be exercised when the 

 reaction is done in this way, since occasional positive results are 

 obtained with cases clinically not syphilitic. These workers believe 

 that cholesterinized antigen is extremely useful, but advise its use 

 only parallel with the ordinary lipoidal antigens and together with 

 careful study of the clinical aspects of the case. 



The fact that these antigens are non-specific in origin naturally 

 necessitates careful determination of their usefulness before they 

 are used. Before any antigen can be regarded as reliable, therefore, 

 a titration must be carried out in the following way : Two series of 

 tubes are prepared, in the first of which antigen and complement are 

 added to normal serum, and in the second the same substances are 

 added to known syphilitic serum. The antigen must, of course, be 

 such that in no test tube does it cause alexin fixation in the presence 

 of normal serum, but, in the quantities used, it must give fixation 

 regularly with syphilitic serum. An example of such a titration 

 may be tabulated as follows : 



EXAMPLE OF ANTIGEN TITRATION 



Antigen by Landsteiner's method: normal guinea pig heart 

 freed from fat and ground up in a mortar. To each gram is added 

 5 c. c. of absolute ethyl alcohol and the mixture allowed to extract 

 at 60 C. for 12 hours (or several days at 37.5 C.). It is then fil- 

 tered through paper. The following titration is then carried out : 



The volume in all of these tubes is brought to 3 c. c. with isotonic 

 salt solution. After one hour at 37.5 C., sensitized red cells are 



