192 CIIEMLSTIx'V OF THE JMMUMTY KEACTI0N8 



precipitate consists of more than the precipitin alone ; it ma}'' be added 

 that the precipitate is always less in amount than the total giobuliu of 

 the antiserum."'^ It is always greater when the reaction is between 

 homologous antiserum and antigen, than with even closely related but 

 heterologous antigens,^' so that the quantitative measurement of the 

 amount of i)recipitate is of value in applying this reaction to deter- 

 mine the imture of protein solutions. The dilution of the reacting 

 solutions is of influence, however, for if in too dilute solutions weak 

 precipitins may fail to give reactions; with strong precipitins the 

 influence of dilution is nuich less (Michaelis). 



According to the source of the protein used we recognize bacterial 

 precipitins, phyto-precipitins (for plant proteins),'*^ and zooprecipi- 

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

 and exudates are generally used in immunizing, i)urified constitu- 

 ents of these protein mixtures will also excite precipitin formation, 

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

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

 cipitability and their power to produce precipitins, the former prop- 

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

 slowly. Heating to coagulation — indeed, heating in the autoclave — 

 does not destroy the precipitinogenous property of proteins, but modi- 

 fies somewhat the reactions of the precipitin obtained,^ and precipi- 

 tinogen is destroyed by alkalies. The specificity of precipitinogens is 

 so modified by heating that the precipitins engendered by a boiled 

 antigen react with the boiled antigen and with similarly heated anti- 

 gens from other species, but not with 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 their 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 



06 Francescliclli, Arch. f. llyg., ]!)07 (00). 207. 



07 Welsh and Cliapman, .Toiir. Hygieno, 1910 (10), 177. 



98 Literatures on precipitins for vegetable proteins given bv Wells and Osborne, 

 Jour. Infect. Dis., 1!)11 (8), 66. 



1 See Oberniayer and Pick, who consider in detail the efVects of various modilica- 

 tions of proteins upon their ))o\ver to incite precipitin formalion (Wien. kiln. 

 Woch., 1006 (19), 327). The precipitability of the serum, or its power to pro- 

 duce precipitins, is not affected by disease "(Pribram, Zeit. exp. Path. u. Ther.. 

 1006 (3). 28). 



-'Schmidt, Bioclieiu. Zeit., 1908 (14), 294; 1910 (24). 4.".; Zeif. lininunitiil., 

 I!tl2 (i:i), 173; also Zinsser, "Infection and Resistance," l!»14. p. 2t;(). 



••'.Jour. Kxper. Med., 1912 (15), 529; 1913 (18), 219. 



