DISEASE, INFECTION, AND RESISTANCE 



291 



teria are to be seen, the cells being arranged in large clumps. 

 In the macroscopic test blood serum is added to a broth 

 culture of typhoid bacilli or to a suspension of the typhoid 

 bacilli in a physiological salt solution. When allowed to 

 stand for a suitable length of time, the bacteria will floc- 

 culate out leaving a clear, supernatant liquid and a floc- 

 culent sediment. 



Agglutinins apparently are not produced in quantities 

 sufficient for assistance in recognition of many of the com- 

 mon diseases. The agglutination test is used most fre- 



FIG. 60. AGGLUTINATION. Left: Unagglutinated bacteria. Bight: 

 Clumped or agglutinated bacteria. 



quently in typhoid, paratyphoid, dysentery, glanders, and 

 for the differentiation of the various types of bacteria 

 capable of causing pneumonia and meningitis. 



Protein Precipitation. Like the agglutinins the pre- 

 cipitins are highly specific. Precipitins developed for one 

 kind of protein will usually not cause precipitation of other 

 types. This property of precipitin formation upon the 

 introduction of foreign protein has been used for the dif- 

 ferentiation of proteins in several instances. By the injec- 

 tion of a particular protein into a suitable animal it is 

 entirely possible to secure from that animal a serum which 

 can thereafter be used as a specific test for the protein 

 injected. This method is used in practice in the differen- 



