240 INFECTION AND IMMUNITY. 



Precipitate. The precipitate which is caused when a bacterial 

 filtrate is mixed with its specific antiserum forms 

 in from one-half hour to several hours, and appears 

 as a coherent white sediment which in the course 

 of twenty-four hours has left the overlying fluid 

 quite clear. The action of the precipitins for 

 serums is more rapid, and in either case sedimen- 

 tation is hastened by placing the fluids at body 

 temperature. As intimated above, the occurrence 

 of the reaction depends on an intact condition of 

 the coagulin groups of both substances. A low 

 concentration of organic acid favors, whereas min- 

 eral acids and alkalies inhibit or prevent precipi- 

 tation ; a neutral reaction is indifferent. The pre- 

 cipitate contains albumin, which, however, has be- 

 come so changed that it is not susceptible to the 

 action of trypsin. The two in combining have in 

 some way shut off the point of attack for trypsin. 

 A lactoserum precipitates the casein of the corre- 

 sponding milk. The presence of salts is necessary 

 for the reaction of precipitation. Both agglutinin 

 and agglutinogen are present in the precipitate, 

 but there seems to be no law governing the 

 amounts of each in the combination. 



The supernatant fluid contains a remaining 

 soluble part of both substances as can be shown by 

 adding fresh precipitin and vice versa. 



Group Pre- Group precipitation is not so pronounced as 

 group agglutination, yet it exists to a certain de- 

 gree and is of the utmost practical importance in 

 attempting to differentiate serums by the precipi- 

 tation method. Although bacterial precipitins are 

 highly specific, it is important to observe the prin- 

 ciple of serum dilution which was emphasized 

 under agglutination, in order to obtain the adven- 



