Antigen for Precipitation Test 



491 



This titration shows that in Procedure I two and a half parts of 

 salt solution added to one part of antigen represent approximate- 

 ly the smallest amount of salt solution which will produce an 

 opalescent mixture. The antigen-salt solution titre of this par- 

 ticular antigen, therefore, is 



Antigen : Salt Solution = 1 :2.5 

 In Procedure II, on the other hand, one part of salt solution 

 added to one part of antigen appears to represent approximately 

 the smallest amount of salt solution which produces a precipitate 

 capable, after being freed from supernatant fluid, of forming 

 an opalescent suspension in salt solution. The titre, therefore, is 

 Antigen: Salt Solution = 1 :1 



It is evident that the antigen titration outlined gives only ap- 

 proximate amounts of salt solution which produce opalescent mix- 

 tures. Occasionally one will find on trial that these amounts of 

 salt solution can be slightly lessened and still obtain opalescent 

 mixtures. Under these conditions, the smaller amount of salt 

 solution should be employed in the tests. 



After determining the smallest amounts of salt solution to 

 mix with antigen in Procedures I and II, the next step is to test 

 the opalescent mixtures with at least six positive and six nega- 

 tive sera, and particularly establish that there is no tendency for 

 weak non-specific reactions. 



One will not encounter non-specific reactions when employing 

 opalescent mixtures. One should not confuse occasional sedi- 

 ments or precipitates settled on the bottom of the tube for 

 specific precipitates which are almost invariably suspended in the 

 medium. The employment of serum controls (0.3 c.c. serum + 

 0.05 c.c. salt solution) will overcome the uncertainty regarding 



TABLE II. EESULTS OP ANTIGEN TITRATIONS 



PROCEDURE I | PROCEDURE II 



Proportion of antigen and salt solution producing 

 opalescent mixtures 



Antigen: Salt Sol'n. | Antigen: Salt Sol'n. 



Non-Cholesterinized an- 











tigen 



1 



: 1.75 



1 



0.85 



Same antigen with 0.4 











per cent, cholesterin.. 



1 



2.5 



1 



1.1 



Same antigen with 0.8 











per cent, cholesterin.. 



1 



3.5 



1 



1.4 



