448 IMMUNOLOGY 



of uniform dispersion for any set of standard conditions. The 

 size of the cholesterol particles may be controlled by the relative 

 volume of alcohol, concentration of cholesterol in the alcohol and 

 the amount of water used as a dispersing fluid. Solution of lipoids 

 gives suspensions of extreme variability, even when mixed under 

 apparently identical conditions. The addition of cholesterol ap- 

 pears to stabilize the dispersion. Production of antigen particles 

 of identical size under laboratory conditions is of great importance. 



Role of Cholesterol. — Eagle (1930) and others believe the role 

 of cholesterol is purely physical in that it determines the size of 

 the dispersed particle. The cholesterol forms the core and the 

 antigenic lipoids cover it witli a thin film. As the size of the 

 particles increases, the total surface area is decreased, requiring 

 less reagin to film the particle. This results in less reagin per 

 unit volume of serum necessary to produce aggregates and auto- 

 matically increases the sensitivity of the test according to Eagle. 

 Other substances which may be substituted for cholesterol in pre- 

 cipitation tests are: solid sterols, lecithins, solid alcohols, balsam 

 of Tolu, salicylic and benzoic acid. 



In some of the new microscopic tests, the synergistic substance 

 is dispersed before the lipoidal extract is added (Kline, Rosen- 

 thal, etc.). The finely precipitated lipoids are adsorbed on the 

 surface of the cholesterol particles. Suspensions made in this 

 manner have particles of greater consistency in size and behavior. 

 The type of suspension differs from those made ])y precipitating 

 the same amounts of lipoids and cholesterol from the same solution. 



All of the synergistic substances used are insoluble, forming 

 suspensions in the finished alcoholic-aqueous menstruum used as 

 a reagent. Such suspensions follow the laws of physicochemical 

 flocculation. This nonspecific chemical flocculation imposes limita- 

 tions on the concentration which can be used to synergize (or 

 sensitize) the lipoids. 



These substances are without antigenic activity when used 

 alone. The exact chemical and physical role of cholesterol or 

 similar substances in the precipitation has not been determined. 



Water. — Water may be added separately as distilled water or 

 as a solvent of some of the reagents, such as salt solution. The 

 water serves as a dispersing medium for the particles of the anti- 



