238 INFECTION AND IMMUNITY. 



ter is unchanged. Hence precipitin, like agglu- 

 tinin, is composed of two groups, a binding or 

 haptophorous, and a ferment-like group in which 

 the active property reside; the latter is the coag- 

 ulin of the molecule. When precipitin has lost its 

 coagulin it becomes precipitoid, and as precipitoid 

 it may unite with precipitogen and thereby inhibit 

 the action of a fresh precipitin which may be 

 added later. When a precipitating serum has 

 partly degenerated into precipitoids, that is, when 

 it consists of a mixture of precipitin and precipi- 

 toid, it is found that the latter have the greater 

 affinity for precipitogen; hence, in concentrated 

 solutions of the serum, precipitoid may be present 

 in sufficient quantity to bind all the available pre- 

 cipitogen, and the reaction would not occur in 

 spite of the presence of active precipitin. This is 

 spoken of as specific inhibition. The action is 

 analogous to that of toxoids and agglutinoids, and 

 the phenomenon is mentioned again in this in- 

 stance in order to emphasize the fact that certain 

 principles of action are common to many of the 

 immune substances. Precipitoids, like toxoids and 

 agglutinoids, are formed by long standing, by the 

 action of heat and light and by other injurious in- 

 fluences. 



The Tnolecule of precipitin, like that of agglu- 

 f;hri n ^ is a receptor of the second order (Fig. 6). 



The attempt has been made to produce antipre- 

 cipitins by immunization with precipitating 

 serums ; this is immunization with an immune 

 serum. It is reported that antibodies have been 

 obtained for lactoserum, but not for bacterial pre- 

 cipitins. There is a limit to the cycle of antibody 

 formation. 



