184 CHEMISTRY OF THE IMMUNITY REACTIONS 



PRECIPITINS" 



If to a solution containing proteins we add in proper proportions 

 the serum of an animal immunized against the same protein, a pre- 

 cipitate will soon form. While not absolutely specific, the quantitative 

 specificity of the precipitin reaction is sufficiently characteristic to be 

 of great value in biological, bacteriological, and medicolegal work, 

 and it is of importance to the phj^siological chemist, since it furnishes 

 a means of distinguishing between closel}^ related forms of proteins, 

 more delicate by far than any known chemical reagent. The serum 

 reactions also prove that there are sometimes essential differences be- 

 tween the proteins of , different species of animals, even when by all 

 other methods these proteins seem to be practically identical; e. g., 

 lactalbumin of cow's milk is in some respect different from lactal- 

 bumin of goat's milk since it produces a different precipitin. Medi- 

 colegally they offer an accurate method of determining the origin of 

 blood and serum stains, no matter how old the stain may be; thus 

 Hansemann'*'- found that material obtained from a mummy 5000 

 years old gave the precipitin reaction.'*^ 



Production of Precipitins. — For the production of the precipi- 

 tation reaction it is necessary to have in the substance used for immu- 

 nization a certain group, the precipitinogen, which when injected gives 

 rise to production of precipitin by the animal. Apparonth' almost 

 any protein may act as a precipitinogen if injected into the proper ani- 

 mal, but it must he a foreign protein; rabbit serum will not pioduce 

 precipitins if injected into a rabbit, "*•* probably because it is normally 

 present in the, blood of the rabbit and therefore does not stimulate 

 any reaction; but certain chemical alterations in the proteins of an 

 animal, such as hegiting, iodizing, or partial digestion, may render them 

 so different from the normal proteins of the same animal that they will 

 act as an antigen when present in the blood of that animal, or another 

 of the same species, from which they were derived. Of the natural 

 proteins of serum the globulins arc much more active precipitinogens 

 than the albumins. In general the more foreign the protein, the 

 greater the amount of precipitin; closely related animals, e. g., rabbit 

 and guinea-pig, produce little precipitin for one another's proteins. 

 This indicates distinctly that difference in species depends upon orj is 

 associated with difference in chemical composition of the proteins. 

 Different species of animals have very different capacity for produc- 

 ing precipitins, rabbits producing active sera, while guinea-pigs can 



^' For complete l)ibliogiaphy of the suliject of "Precipitins" see the r('>suin(' by 

 Michaclis, Ujjpenheinier's llaiull). il. Hiochcinic. I'.IOK, II (1), bi>'2; Kraus, KoUc 

 and Wassiirinann's llandb., 1913, II; Llilenliiitli and Stel't'enliagen, ibid., Ill, 257; 

 Zinsser, "Infection and Resistance." 



« Munch, nied. Woch., 1904 (30), 572. 



" Not corroborated by Schnudt, Zeit. allg. Pliysiol.. 1907 (7), 309. 



^* Rarely a slight reaction against homologous i)roteins has been obtained {iso- 

 precipitins). 



