THE PHENOMENON OF PRECIPITATION 249 



the discovery of Kraus was developed into the generalization that 

 the sera of animals that have been treated with foreign proteins of 

 any kind bacterial, animal, or vegetable will develop the property 

 of causing precipitates when mixed with clear solutions of the re- 

 spective antigens. 



The substances which, after injection into the animal body, lead 

 to the formation of precipitating antibodies are spoken of in the 

 language of immunology as "precipitinogen." In the case of bac- 

 teria it has been shown that, while the injection of the whole bac- 

 terial cell dead or alive will lead to precipitin formation, bacterial 

 extracts produced in a variety of ways will lead to the same result. 

 Such precipitinogen extracts can be obtained by allowing the bacteria 

 to grow in flasks of slightly alkaline bouillon, keeping them in the 

 incubator for from three weeks to three months, and then filtering 

 them through Berkefeldt candles. Again, useful extracts can be 

 more rapidly produced by growing large quantities of bacilli on 

 agar, emulsifying in salt solution, and shaking in any one of the 

 ordinary types of shaking machine for 48 hours or longer. On filter- 

 ing an extract is obtained which will form precipitates with homol- 

 ogous immune serum, or will incite precipitins when injected into 

 animals. In fact, any one of the customary vigorous methods of 

 extracting bacterial or other cells will yield precipitinogen. A rela- 

 tively purified precipitinogen in the form of a dry, water-soluble 

 powder has been obtained by Pick by the precipitation of culture 

 filtrates with alcohol. 



Regarding the chemical nature of the precipitin-inducing sub- 

 stances, or precipitinogens, the same problems have arisen which 

 have been discussed in connection with antigens in general. We may 

 say that all soluble native proteins possess precipitin-inducing prop- 

 erties. Yet this does not sufficiently define the term, since many 

 observations have been published which show that physically and 

 chemically altered proteins may still induce specific precipitins; a 

 few investigators, furthermore, have claimed that they have produced 

 non-protein precipitinogen by various methods of breaking up the 

 molecule of the original antigen. In the section on agglutination 

 we have seen that moderate heating (56-65 C.) rather increases 

 than decreases the agglutinogen characteristics of bacteria, and it 

 is equally true that such heated bacteria or bacterial extracts may 

 induce precipitins. However, regarding the action of higher de- 

 grees of heat (boiling) upon precipitinogens in general we will have 

 more to say in another place. 



Of more immediate, indeed of fundamental, importance is the 

 problem of a non-protein antigen. The most important claims in 

 this regard have been made by Pick, 4 Obermeyer and Pick, 5 and by 



4 Pick. "Hofmeister's Beitrage," Vol. 1, 1901. 



5 Obermeyer and Pick. Wien. klin. Woch., 1904, p. 265. 



