452 IMMUNOLOGY 



The type of reaction depends upon the kind of antigen prepared. 

 Macroscopic reactions require larger particles than microscopic 

 reactions. The larger particles undergo a pseudoagglutination that 

 is easily converted into larger aggregations in the presence of 

 syphilitic serum. Such larger clumps give an easy end-point in 

 reactions like the Kahn. 



On the other hand, microscopic reactions must have antigens 

 with finely divided particles. Such reactions are made on a mi- 

 croscopic slide, and the clumps are magnified from eighty to one 

 hundred diameters. Minute clumps in the negative serum give 

 pseudopositive reactions which interfere seriously with the test. 

 Only slight clumping is necessary for a positive reaction with this 

 magnification. 



Choice of Test. — Numerous tests have l)een ])ublis]ied, and al- 

 though tliey differ in method, the mechanics of each is similar. 

 The choice of a standard flocculation test awaits the accumulation 

 of laboratory and clinical correlation over a long period of time. 

 Until that time the choice of the flocculation test depends chiefly 

 upon personal preference and individual success. Several of the 

 mo.st used of the flocculation tests will be briefly described. 



Kahn Precipitation Reaction (1940). — The antigen consists of 

 ether-insoluble, alcohol-soluble lipoid fractions of beef heart with 

 the addition of 0.6 per cent cholesterol. The antigen is standard- 

 ized by a series of titrations. 



Salt Titration. — This represents the minimum amount of 0.90 

 per cent saline necessary for dispersion of the antigen and that 

 will also permit resolution on addition of 1 c.c. or less of isotonic 

 saline after the emulsion has been shaken for three minutes witli 

 0.15 c.c. of saline and incubated for fifteen minutes in a 37° C. 

 water bath. This is known as the titer of the antigen and repre- 

 sents the amount of saline solution which must be added to 1 c.c. 

 of lipoid solution. 



The procedure is best illustrated by the accompanying protocol 

 of an actual titration (Tables XII, XIII). The protocol is modi- 

 fied after Kahn. 



This gives a heavy white precipitate in each dilution vial. Three 

 tubes containing respectively 0.05, 0.025, and 0.0125 c.c. of anti- 

 gen are prepared for each dilution as in Table XIII. 



