AGGLUTININS AND PRECIPITINS 107 



Fink worked with the precipitates obtained by salting protein solutions 

 and found that rabbits inoculated with one-fourth, one-third, one-half, 

 and two-thirds saturation products produced no precipitins nor com- 

 plement-fixing bodies. In guinea-pigs, however, the three-fourths 

 saturated and completely saturated products possess slight sensitizing 

 and intoxicating properties, the latter being apparently the more 

 active. Nevertheless, three-fourths saturated and completely saturated 

 products of egg-white were sufficient to produce definite formation of 

 precipitin and complement-binding antibodies but not in as high a titer 

 as entire protein. 



Nature of the Reaction. In analogy with the terms used in the 

 phenomenon of agglutination Kraus named the antigen, precipitinogen 

 and the immune body precipitin. The reaction is similar to agglutina- 

 tion in all respects save that here we have to deal with proteins in 

 solution. Aging or heating leads to the formation of precipitoids, 

 group reactions as well as inhibition zones appear, heat has much the 

 same influence in all respects as in agglutination, salts play an im- 

 portant part in the reaction and specific absorption can be demonstrated. 

 It is known, however, that some protein molecules are largely built 

 up of alkaline amino-acids and that others are built up largely of the 

 acid amino-acids. Salmine, for example, a product of the spermatozoa 

 of certain fish, consists almost entirely of strongly alkaline amino-acids. 

 Gliadine of wheat is chiefly built up of dibasic amino-acids, glutaminic 

 acid. The fermentation end product of salmine is alkaline and of gli- 

 adine acid in nature. An antigliadine serum gives with a homologous 

 precipitinogen, a beautiful precipitate, while a mixture of salmine and 

 antisalmine-serum gives no visible precipitate. This would indicate 

 that the alkaline salts are of importance in the actual formation of the 

 precipitins, and we know by simple titration that during the precipitin 

 reaction there occurs a reduction of acidity. Nevertheless, it is also 

 asserted that when the acidity is due to an organic acid or acid salt 

 the reaction appears to be promoted. The precipitin is precipitated in 

 the euglobulin fraction of the serum, is destroyed slowly by trypsin 

 and rapidly by pepsin. The immune serum contains the precipitin 

 which constitutes the bulk of the precipitate, the latter thus represent- 

 ing, according to Wells, " the insoluble modification of the previously 

 dissolved precipitin and originates chiefly or entirely in the proteins 

 of the immune serum." Welch and Chapman obtained, with a precipi- 

 tinogen containing only i gram of protein, a precipitate containing 21.1 

 grams of protein. Pick employed a precipitinogen which did not give 

 the biuret reaction and with this obtained a voluminous albuminous 

 precipitate. It must not be understood that precipitins are always the 

 result of immunization, for Vaughan states that goat serum contains 

 a normal precipitin for rabbit and for dog sera. Such normal pre- 

 cipitins are not of very high titer and are not so sharply specific as 

 the immune precipitin. Puppies, kittens and rabbits up to ten days 

 old may absorb native protein from the milk of the mother which 

 apparently stimulates the formation of precipitins. Sera of human 



