PRECIPITINS 187 



precipitins may fail to give reactions; with strong precipitins the 

 influence of dilution is much less (Michaelis). 



According to the source of the protein used we recognize bacterial 

 "precipitins, phyto-prccipitins (for plant proteins), ^^ and zooprecipi- 

 tins (for animal proteins). Although tissue extracts, body fluids, 

 and exudates are generally used in immunizing, purified constitu- 

 ents of these protein mixtures will also excite precipitin formation, 

 e. g., we may immunize with caseinogen as well as with milk. Com- 

 plete pepsin digestion of proteins deprives them both of their precipi- 

 tabihty and their powder to produce precipitins, the former property 

 being lost first. Trypsin seems to produce the same effect more slowly. 

 Some of the fractions of protein cleavage may be slightly precip- 

 itinogenic (Fink);''" Heating to coagulation — indeed, heating 

 in the autoclave — does not destroy the precipitinogenous property of 

 proteins, but modifies somewhat the reactions of the precipitin ob- 

 tained," and precipitinogen is destroyed by alkalies. The specificity 

 of precipitinogens is so modified by heating that the precipitins en- 

 gendered by a boiled antigen react with the boiled antigen and wuth 

 similarly heated antigens from other species, but not w'ith unheated 

 antigens even from the homologous species.''- 



As proteins introduced into the stomach are normally destroyed 

 before being absorbed, they do not enter the blood and cause pre- 

 cipitin formation. However, as is well known, eating of excessive 

 amounts of egg-albumen or other easily absorbed proteins may re- 

 sult in theii' passing the barriers and entering the blood stream, and 

 in this way precipitins have been experimentally produced. Pre- 

 sumably the precipitin reaction is a means of throwing such foreign 

 proteins out of solution and rendering them harmless. According 

 to Zinsser''^ and others, the function of the precipitin is to sensitize 

 the unformed foreign proteins to the digestive complement, a view 

 in harmony with the prevailing tendency to correlate the immunity 

 reaction with defense through enzymatic hydrolysis. 



Precipitin appears in the blood generall}' about six days after in- 

 jection of the protein, but disappears after injection of each subse- 

 quent dose of protein, to reappear again after a somewhat shorter 

 lapse^of time. After injections are stopped, the precipitin disap- 

 pears rather rapidly, but never appears in the urine, although it 



°^ Literature on precipitins for vegetable proteins given by Wells and Osborne, 

 Jour. Infect. Dis., 1911 (8), 66. 



«" .Jour. Infect. Dis., 1919 (25), 97. _ 



^' See Obermayer and Pick, who consider in detail the effects of various modi- 

 fications of proteins upon their power to incite precipitin formation (Wien. klin. 

 Woch., 1900 (19), 327); also Landsteiner and Lampl, Biochem. Zeit., 1918 (86), 343. 

 The precipitability of the serum, or its power to produce precipitins, is not affected 

 by disease (Pribram, Zeit. exp. Path. u. Then, 1906 (3), 28). 



«2 Schmidt, Biochem. Zeit., 1908 (14), 294; 1910 (24), 45; Zeit. Immunitat., 

 1912 (13), 173; also Zinsser, "Infection and Resistance," 1914, p. 260. 



" Jour. Exper. Med., 1912 (15), 529; 1913 (IS), 219. 



