AGGLUTININS AND PRECIPITINS 109 



hours, filtered through a Berkefeld filter and the filtrate used. Kraus, 

 in his original studies, used broth filtrates and also juice expressed 

 from the bacteria. Kraus points out that the broth filtrates of toxin- 

 producing organisms such as bacillus diphtheriae do not precipitate 

 when mixed with antitoxic serum. That this is a general rule, how- 

 ever, is not true, since Jacoby has shown that it is possible to obtain a 

 precipitate by mixing ricin and antiricin serum, and others have ob- 

 served similar reaction with the use of abrin and antiabrin serum as 

 well as crotin and anticrotin serum. 



The delicacy of the precipitin reaction is great and only exceeded, 

 in certain respects, by complement fixation and the anaphylaxis reac- 

 tion. It is of interest to note that whereas the Biuret and the Millon 

 test for protein will hardly exceed dilutions of i-iooo, the precipitin 

 reaction will detect not only the presence of protein but the species 

 from which it originates, commonly in dilutions of 1-10,000 or 1-20,000 

 and even 1-100,000. 



Physical Basis of Precipitation. The influence of heat on pre- 

 cipitation and also the group reactions are of considerable importance in 

 the practical application of the phenomenon and will be dealt with more 

 fully as this side of the question is considered. The comparisons offered 

 between agglutination and certain colloidal phenomena (see page 94) 

 are equally applicable to precipitation and require no extensive dis- 

 cussion here. It must be borne in mind, however, that the colloidal 

 interpretation of these phenomena is not proven. Essentially the same 

 arguments are available against the conception of precipitoids as 

 against that of agglutinoids, but none of these explains satisfactorily 

 the specific absorptive capacities of these hypothetical bodies. As ag- 

 glutinogen and agglutinin may exist in the blood of a living animal, 

 so may precipitinogen and precipitin coexist. This is compared by 

 Zinsser to the fact that if gum arabic is added to a mixture of thin 

 gelatin and arsenic trisulphide the precipitation which ordinarily occurs 

 will be prevented. The gum arabic in this instance is a protective 

 colloid. It is assumed that such a protective colloidal action operates 

 to prevent precipitation when precipitinogen and precipitin coexist in 

 the blood of a living animal. After the blood is withdrawn and 

 allowed to stand, this protective action disappears and precipita- 

 tion occurs. 



The fact that precipitin and precipitinogen can coexist in circulating 

 blood and that experiments on the attempted production of iso- 

 agglutinins with their conflicting results has led to the question of 

 whether or not it is possible to produce precipitins in an animal by 

 the injection of proteins of a closely related species. Uhlenhuth and 

 Weidanz claim to have produced precipitins for human serum by 

 injecting human serum into monkeys, the resulting precipitin acting 

 on human but not on monkey serum. Berkeley and later Sutherland 

 were unable to confirm this experiment and we are forced to the con- 

 clusion that precipitin formation in closely related species is by no 

 means a constant phenomenon. Such precipitins would be practically 



