264 'BIOLOGY OF SPECIALIZED GROUPS 



The normal opsonins may be increased by immunizing the 

 animal body with killed bacteria, thus causing the cells of the 

 body to react and secrete more opsonin, as is the case in anti- 

 toxin formation. 



Precipitins. If the blood serum of an animal which has 

 recently had an infection, or been artificially immunized, is 

 brought in contact with a filtrate of bacterial culture of the 

 same species of bacteria which caused the infection, or the 

 species which was used in the process of immunization, a 

 definite chemical reaction results. This reaction is char- 

 acterized by the precipitation of certain of the protein sub- 

 stances in the filtrate which are evidently derived from the 

 bacterial cells. The substances in the serum are called pre- 

 cipitins. The reaction is nearly a specific one ; that is, pre- 

 cipitation is rarely accomplished by a serum when placed in 

 contact with a filtrate produced by bacteria other than those 

 used in the process of immunization, or present in the infec- 

 tion. Occasionally, for example, the serum of an animal 

 immunized to B. typhosus will precipitate the filtrates of 

 other members of the so-called Typhoid-Colon Group, such 

 as B. coli, B. paratyphosus, etc. This fact indicates that 

 there is a close chemical relationship between these bacteria. 

 Specific precipitins have been obtained for practically all 

 protein substances such as blood, milk, and egg albumen 

 with serums produced by immunizing animals with these 

 foreign substances. Precipitins are also specific for the bloods 

 of various animals. Precipitins are distinct from agglutinins 



although they are both frequently present in the same serums. 



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