PRECIPITINS 233 



antibody, at times, eoexist in the blood without the formation of 

 a precipitate is the absence of optimal ratio of concentrations of 

 each. 



Nature of Precipitate. — It is generally conceded (Dean, 1931, 

 Zinsser, 1931) that the major portion of the precipitate is made 

 np of serum globulin from the immune serum. 



Effect of pH on the Reaction.^ — The etfect of pH on the forma- 

 tion of immune precipitates has been investigated by Hirsch 

 (1923). Using sheep serum and antisheep serum he found that 

 precipitation occurs over a wide range, i.e., pH 6 to 9.4 with the 

 maximum precipitation occurring near the acid end of the range. 

 Mixtures having an acidity greater than pH 6 showed nonspecific 

 precipitation, while precipitation was inhibited when the reac- 

 tion was more alkaline than pH 9. Neutralization of the mix- 

 tures restored the ability to give specific precipitates. 



Effect of Salts on the Reaction. — The effect of varying the con- 

 centration of sodium chloride used in the diluent was studied by 

 Nuttall (1904), Dean and Webb (1926), Hektoen (1928), Baier 

 (1933) and others. Dean and Webb found that the optimal pro- 

 portions of antigen to antiserum did not vary when concentra- 

 tions of sodium chloride varying from 0.02 to 1.0 per cent were 

 used although the speed of the reaction was affected by the con- 

 centration of the salt. In their opinion a concentration of 0.2 

 per cent NaCl was the most favorable. As the concentration of 

 salt was increased, the reaction was slower. Hektoen recom- 

 mends that where immune rooster serum is used sodium chloride 

 concentrations of 1.8 per cent be employed to eliminate non- 

 specific reactions. Recently Downs and Gottlieb (1932) have 

 studied the effects of different electrolytes upon the formation of 

 precipitates. They* found that "certain salts in molar solution 

 inhibit the formation of a precipitate from horse serum and anti- 

 horse rabbit serum, but others do not. All the salts studied in- 

 hibit the formation of a precipitate from crystalline egg albumen 

 and its antiserum when present in molar solution." They state 

 that some additional factor besides the valence is of importance in 

 determining precipitation. They found that the thiocyanates 

 showed the maximum inhibitory eff'ect. In regard to the peptizing 

 effect of the thiocyanate, they say, ' ' The thiocyanate ion can there- 



♦Downs, C. M., and Gottlieb, S. : J. Infect. Dls. 51: 460, 1932. 



