446 IMMUNOLOGY 



"antigen'' used to detect syphilitic reagin consists of a disper- 

 sion of lipoidal substances in an aqueous suspending fluid (con- 

 tinuous phase). The reaction between the reagin in the serum 

 and the particle of the "antigen" (the discontinuous phase) oc- 

 curs at the surface interface of the particle. There is deposited 

 a film of "antibody globulin" around the particle with a corre- 

 sponding alteration in solubility of the interfacial film through 

 an "antigen-antibody complex" formation with behavior like a 

 denatured protein. This results in a reduction of the potential 

 difference between the particles,, when electrolytes are present 

 in the aqueous phase. The mixture is agitated in some mechanical 

 manner so that tlie particles come in contact with one another. 

 These particles arc held together, after contact is once made, by 

 their coliesive force in the absence of a high potential difference. 

 A high potential difference Avonld cause the charged particles to 

 repel each other. 



In the negative reaction no clumping of particles occurs. There 

 are no changes in the P.I), and the like charged particles repel 

 one another with no resulting clianges in dispersion. 



The agglutinated particles may vary from a finely granular 

 microscopic precipitation to a dense macroscopic precipitate, de- 

 pending upon the composition of the tissue extract, the reagin 

 titer of the syphilitic serum and the technique of the test. Thus 

 precipitation tests may vary from a microscopic slide test to a 

 macroscopic test tube reaction. 



Role of Ingredients. — The sensitivity and specificity of the pre- 

 cipitation reaction is largely determined by the following: 



(1) Fraction of total lipoids extracted from heart muscle. 



(2) Synergistic (or sensitizing) substances added to tissue 

 extract. 



(3) Electrolyte concentration. 



(4) Lipoidal concentration. 



(5) Lipoidal-cholesterol ratio. 



(6) Alcohol- water ratio. 



With the data available, it is impossible to evaluate the impor- 

 tance of each, nor can the relative interdependence of all or 

 several be calculated Avith success. In order to simplify an ele- 



