PRF.CIl'ITIX TKSTS IxV SYPHILIS 447 



mentary presentation of proeipilalion lesls, the various ingredi- 

 ents and ratios of ingredients will be discussed as separate topics. 



Antigens. — The term "antigen" is a misnomer applied to the 

 concentrated lipoidal tissue extracts used as a reagent in this test. 

 However, in keeping with usage, the word antigen has been used 

 in this chapter to designate tissue extracts. Tiie antigen used in 

 flocculation tests is composed of a portion of tlie lipoids of normal, 

 rapidly dried, fresh heart muscle. Beef heart, freed of connective 

 and fatty tissue, is generally used for this purpose. Any chemical 

 compound in which lipoids are soluble and proteins are insoluble 

 can be used as an extracting fluid. Several extracting fluids may 

 be used as the total lipoid content is not soluble in any one solvent. 

 Further, solvents may be chosen to remove undesirable lipoidal 

 fractions (as ether in preparation of Kahn antigen) and the dried 

 residue extracted with other solvents for the '^antigenic lipoids." 

 In general, the ether-soluble lipoids are characterized by their in- 

 stability in water dispersions, while the ether-insoluble, alcohol- 

 soluble lipoids give relatively stable suspensions. 



The extracted lipoids may be separated into acetone-soluble 

 and acetone-insoluble fractions. The acetone-insoluble fractions 

 of the alcohol-soluble portion have given very satisfactory results 

 in microscopic tests. 



The character of the extract can again be modified by chang- 

 ing the temperature of the solvents during extractions. Each of 

 the fractions of lipoids appears to differ slightly in chemical com- 

 position and activity but they grade into one another like colors 

 of the spectrum. Hence any one fraction represents a series of 

 lipoids. Chemical analysis has failed to reveal the structural 

 formula or even the active compounds. The extractives appear 

 as a light yellow to a dark brown wax.v substance composed of 

 lipoids of the cholesterol and lecithin series along with a larger 

 undetermined fraction known as tissue extractives. 



Synergistic Substances. — Substances added to the alcoholic 

 lipoidal extract which increase the sensitivity of the finished 

 antigen are known as sj'iiergizing (or sensitizing) agents. These 

 substances are compounds of known chemical purity, organic in 

 nature and soluble in fat solvents but insoluble in water. The 

 most important of this group is cholesterol. Cholesterol in alco- 

 holic solution is precipitated by addition of water, giving crystals 



