316 PRECIPITINS 



of monkeys. Similar reactions were found to occur among the lower 

 animals. 



Nomenclature. The antibody in an immune serum responsible for 

 the phenomenon of precipitation is called precipitin; the substance or 

 antigen responsible for the production of this antibody is known as the 

 precipitinogen; the precipitate is the end-product of the reaction between 

 precipitinogen and precipitin. Just as toxoids and agglutinoids may be 

 formed, so precipitin may be modified to precipitoid. 



Although, since bacterial precipitins are produced by the protein 

 constituents of bacteria, the custom of differentiating between bacterial 

 and protein precipitins is superfluous, nevertheless, when the mean- 

 ing is clearly understood, the term bacterial precipitin is convenient 

 and may be employed. 



The precipitins derive their names from their precipitinogens, as, 

 for example, a precipitin produced by injecting rabbits with ox serum 

 is designated anti-ox precipitin. 



Normal Precipitins. Although agglutinins may be found in normal 

 serum, it is decidedly uncommon to find normal precipitins. Extracts 

 of organs have been known to contain normal precipitins for certain 

 albumins, although at the same time they were absent from the serum 

 of the animal. In this case the active bodies exist in the cells as "ses- 

 sile receptors," and by the process cf extraction they are brought into 

 solution. During immunization these same receptors are stimulated to 

 overproduction and are thrown into the circulation as free precipitin 

 receptors. 



Immune precipitins are antibodies produced by immunization with 

 a foreign albumin, either during the course of a bacterial infection or as 

 the result of artificial inoculation. 



Structure and Properties of Precipitins. According to the side-chain 

 theory, precipitins are antibodies or receptors of the second order, com- 

 posed of a combining arm or haptophore group for the precipitinogen, 

 and a zymophore or precipitinophore group that precipitates the antigen. 

 Their structure is, therefore, seen to be quite similar to that of agglu- 

 tinin, the difference being largely due to the different functions of the 

 zymophore group. 



The properties of precipitins are quite similar to those of agglutinins. 

 They are fairly resistant bodies, resist the effect of drying for prolonged 

 periods, but are gradually destroyed by heating to 60 to 70 C. When 

 inactivated by exposure or heat, they cannot be reactivated by the addi- 

 tion of fresh normal serum, and therefore they bear no relation to the 



