296 PRECIPITINS 



immunization of a guinea-pig with the serum of a rabbit, a pigeon with 

 that of a hen, or a monkey with human serum, are procedures that do not 

 usually yield good precipitating serums. 



Attempts have been made to produce antiprecipitin by effecting 

 immunization with immune precipitating serums. Such attempts have 

 been reported as partially successful with serum and milk, but not with 

 bacterial precipitins. Antiprecipitins possess no practical value. 



Mechanism of Precipitation. Of the various theories advanced to 

 explain the phenomenon of precipitation, none has received so much 

 support experimentally as that advanced by Bordet in explanation of 

 agglutination. 



Colloids may be precipitated by salts, and probably the salts so alter 

 the electric state of colloidal particles that their surface tension is de- 

 creased, and, as a result of this change, neighboring particles coalesce 

 in such quantities as to produce a visible precipitate. Salts are likewise 

 necessary for serum precipitation, and there is a close analogy between 

 serum and colloidal precipitation. 



The origin of the precipitate formed during the reaction is of interest. 

 When a very potent immune serum is employed, the precipitinogen is so 

 highly diluted that it no longer gives any of the chemical reactions for 

 proteins, but when the precipitating serum is added, it may yield, never- 

 theless, a heavy precipitate. The precipitate can, therefore, hardly be 

 regarded as due to the slight trace of albumin in the precipitinogen, and, 

 furthermore, if the precipitating serum is diluted, the precipitate be- 

 comes smaller and smaller, and, if the dilution is increased, it finally 

 disappears altogether. For this reason the precipitate is generally 

 considered as originating in the immune serum. 



Specificity of Precipitins. Precipitins react but feebly on closely 

 related albumins of the same species, but are specific against those of 

 unrelated species. In other words, the precipitation test merely deter- 

 mines the animal species from which the proteid originates, but cannot 

 demonstrate positively whether it comes from the blood, the semen, 

 milk, or other albuminous body. For medicolegal purposes, therefore, 

 a diagnosis of "human blood-stain" cannot be made without chemical 

 evidence to prove that the stain actually consists of blood. 



An immune serum prepared by the injection of the serum of a certain 

 animal gives a precipitate also with the juices of the various organs of 

 that animal. The only exception to this rule is the protein of the crystal- 

 line lens of the eye, which gives no precipitate with the antiblood im- 

 mune serum. The same albumin exists in the crystalline lenses of all 



