PRECIPITINS 191 



antigen seems to be altered appreciably by the reaction, since when 

 either is separated from tlie precipitate it retains its original prop- 

 erties. 



Since precipitation of colloids is accompanied by or dependent 

 upon an aggregation of their particles, the precipitin reaction is 

 closely related to the agglutination reaction. The amount of precip- 

 itation obtained is much modified by the amount of inorganic salts 

 present, and, according to Friedemann,'''^ there is a general resem- 

 blance between the precipitin reactions and the precipitations occur- 

 ring when colloids precipitate one another; i. e., when an amphoteric 

 colloid reacts with either an acid or a basic colloid.'** So far, however, 

 attempts to interpret the precipitin reaction, as Arrhenius has tried 

 to do, on the basis of the laws of physical chemistry, have not met 

 with much success (Michaelis). We prefer the attitude of Krogh,"*^ 

 who states that the colloidal chemical part of immunological reactions 

 is to be looked upon as only a preliminaiy step to the real chemical 

 process that completes the reaction and gives it the specific characters. 

 As mentioned in the preceding section, agglutination of bacteria is be- 

 lieved to be 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 protein 

 molecule, and another group by which they produce the change re- 

 sulting in precipitation. When the latter group is destroyed by 

 heating to 72°, the precipitin is converted into a precipitoid, which 

 possesses the property of preventing the precipitation of the specific 

 antigen by unheated precipitin.^*'' 



The immune serum contains the precipitin, which is the passive 

 reagent that is throw^n down by a trace of the immunizing material 

 (precipitinogen). The resulting precipitate is the insoluble modifica- 

 tion of the previously dissolved precipitin, and originates chiefly or 

 entirely in the proteins of the immune serum, ''^ according to the work 

 of Welsh and Chapman, especially. But as the precipitate is able to 

 sensitize anaphylactically, both actively and passively, it would seem 

 that it must contain both the antibody (which confers passive sensi- 

 tization) and antigen, to cause active sensitization (Weil).'''^'^ The 

 precipitate may, when of maximum amount, contain more nitrogen 

 than corresponds to the entire euglobulin of the immune serum, and 

 the euglobulin contains all the precipitin, so it seems probable that the 



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



94 See Friedemann and Friedenthal, Zeit. exp. Path. u. Ther'., inOG (3) 73; 

 Iscovesco, Compt. Rond. >Soc. Biol., 1906, Vol. 61, and subsequent volumes. 



94a .Tour. Infect. Dis., 1916 (19), 452. 



94b Precipitinogens are relatively resistant to moderate heating, and heat«d 

 extracts of bacteria are used for precipitin tests imder the name thermoprecipi- 

 tins. See review by A. Ascoli, Virchow's Arch., 1913 (213), 182. 



95 Moll, Zeit., exp. Path. u. Ther., 1906 (3), 325; Welsh and Chapman, Proc. 

 Royal Soc., B., 1908 (80), 161; Zeit. Immunitat., 1911 (9), 517. 



95a Jour. Immunol., 1916 (1), 35. 



