PRECIPITINS, 237 



is active against its own serum, i. e., by bleeding 

 an animal and reinjecting the serum a specific pre- 

 cipitin is not formed. If formed it would be an 

 autoprecipitin, and, as a rule, animals do not form 

 antibodies for their own tissue constituents. 

 Again, animals are less likely to form antibodies 

 for the tissue constituents of other members of the 

 same species than for those of other species ; these, 

 when formed, are called iso-antibodies. Schutze 

 immunized thirty-two rabbits with serum from the 

 rabbit and obtained an iso-precipitin from only two 

 of the number. In the third place, animals do not 

 readily form anti-bodies for the tissue constituents 

 of other animals which zoologically or biologically 

 are closely related. Immunization of the guinea- 

 pig with the serum of the rabbit, a pigeon with 

 that of a chicken, or a monkey with human serum, 

 are procedures which usually do not yield precipi- 

 tating serums. 



Chemically, little is known of precipitins. They Na ture of 

 are thrown down by ammonium sulphate in con- Precip 1 * 1 " 

 junction with the euglobulin fraction of serum, and 

 are destroyed by those substances which alter al- 

 buminous bodies, as acids, alkalies, pepsin and 

 trypsin. That bacterial precipitins are not iden- 

 tical with agglutinins for the same bacteria is 

 shown by the following facts : Immunization with 

 certain bacteria may produce agglutinin but no 

 precipitins. Precipitins develop more slowly than 

 agglutinins. As a rule precipitins are inactuated 

 at lower temperatures than agglutinins. 



When serum is heated to from 50 to 60 C. its specific 

 ability to cause a precipitate in the homologous 

 precipitogen is destroyed, although it may be dem- 

 onstrated that the power to combine with the lat- 



