154 CHEMISTRY OF IMMUNITY AGAINST BACTERIA 



upon aii aggregation of their particles, the precipitin reaction 

 is closely related to the agglutination reaction. The amount 

 of precipitation obtained is much modified by the amount of 

 inorganic salts present, and, according to Friedmann, 1 there is a 

 general resemblance between the precipitin reactions and the 

 precipitations occurring when colloids precipitate one another ; 

 i. e., when an amphoteric colloid reacts with either an acid or a 

 basic colloid. Possibly the constituents of the nucleoproteids 

 furnish the acid and basic colloids for these reactions. 2 As 

 mentioned in the preceding section, agglutination of bacteria is 

 independent of the precipitins, although very probably influenced 

 by them. As with all the other substances of this class, the 

 precipitins have a haptophore group by which they unite to the 

 proteid molecule, and another group by which they produce the 

 change resulting in precipitation. When the latter group is 

 destroyed by heating to 60 , the precipitin is converted into a 

 precipitoid. 



According to the source of the proteid used we recognize 

 bacterial precipitins, pJiyto-precipitins (for plant proteids), and 

 zooprecipitins (for animal proteids). Although tissue extracts, 

 body fluids, and exudates are generally used in immunizing, 

 purified constituents of these proteid mixtures will also excite 

 precipitin formation, e. g., we may immunize with caseinogen as 

 well as with milk. Immunization with frequently recrystallized 

 albumins is less successful (Obermayer and Pick). Complete 

 pepsin digestion of proteids deprives them both of their precipi- 

 tability and their power to produce precipitins, the former 

 property being lost first. Trypsin seems to produce the same 

 effect more slowly. Heating to coagulation indeed, heating in 

 the autoclave does not destroy the precipitogenous property of 

 proteids, but modifies somewhat the character of the precipitiu 

 obtained. 3 



As proteids introduced into the stomach are normally des- 

 troyed before being absorbed, they do not enter the blood and 

 cause precipitin formation. However, as is well known, eating 

 of excessive amounts of egg-albumen or other easily absorbed 

 proteids may result in their passing the barriers and entering 

 the blood stream, and in this way precipitins have been experi- 



. f. Hyg., 1906 (55), 361. 



2 See Friedmann and Friedenthal, Zeit. exp. Path. u. Ther., 1906, (3) 73. 



3 See Obermayer and Pick, who consider in detail the effects of various 

 modifications of proteids upon their power to incite precipitin formation (Wien. 

 klin. Woch., 1906 (19), 327). The precipitability of the serum, or its power 

 to produce precipitins, is not affected by disease (Pribram, Zeit. exp. Path. u. 

 Ther., 1906 (3), 28). 



