186 CHEMISTRY OF THE IMMUNITY REACTIONS 



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 preliminary 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 wdth 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 unheated 

 precipitin by the specific antigen.^'* 



The immune serum contains the precipitin, which is the passive 

 reagent that is thrown 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 

 precipitate consists of more than the precipitin alone; it maj^ be added 

 that the precipitate is always less in amount than the total globulin 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 precipitate is of value in applying this reaction to deter- 

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

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



" See Friedemann and Friedenthal, Zeit. exp. Path. u. Ther., 190G (3) 73; 

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



" Jour. Infect. Dis., 191G (19), 452. 



^* Precijntinogens are relatively resistant to moderate heating, and heated 

 extracts of bacteria are used for precipitin tests under tlie name thermoprccipiiiiis. 

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



" Moll, Zoit., exp. Path. u. Ther 1900 (3), 325; Welsh and Chapman, Proc. 

 Royal Soc, B., 190<S (SO), Kil; Zeit. Immunitat., 1911 (9), 517. 



"Jour. Immunol., 191() (1), 35. 



" Francosclielli, Arch. f. Hvg., 1907 (09), 207. 



" Welsh and Chapman, .Jour. Hygiene, 1910 (10), 177. 



