114 PRECIPITINS. 



It is advisable to inject five or six animals at the same time, instead of only one, inas- 

 much as rabbits vary greatly in their individual power to produce precipitins and more- 

 over, because some die after the third injection. Frequently only one serviceable serum 

 is obtained, even though the immunization of five rabbits was undertaken. 



Beginning on the sixth day after the injection, one should, at regular 

 intervals of one or two days, remove a small quantity of blood from the vein of 

 an ear and test the strength of the serum. As soon as it is found to be 

 satisfactory the animal should be bled and its serum preserved on ice, with 

 precautions for sterility. The rules given above for obtaining a clear serum 

 should be kept in mind. 



If the serum is not withdrawn at the proper time, its strength begins to diminish and 

 further injections no longer stimulate new antibodies. It is even possible for the entire 

 precipitin action of the serum to disappear. 



The following method of titration is the simplest. One c.c. 

 Titration. 



of various dilutions (i :io, i : 100, i : 1000, i : 10000) of the pro- 



teid under examination (precipitinogen) is placed into different test-tubes 

 and o.i c.c. of the precipitating serum is added to each. The tubes should 

 not be shaken, but it is occasionally necessary to place them into the incu- 

 bator for one hour before any turbidity or precipitate appears. The least 

 amount of proteid solution which still distinctly shows a precipitate, is 

 taken as the titer of the serum. 



For medico-legal purposes, Uhlenhuth advises the use of only 



Uhlenhuth's hi g hl 7 valent sera - 



Method of He considers an antiserum as efficient if o.i c.c. of it, when 

 Proteid Dif- mixed with its respective serum in the dilution of 1:1000, 

 ferentiation. produces a distinct turbidity, either at once or one to two min- 

 utes at the latest; three to five minutes is the limit for an 

 indication of turbidity in the dilutions of i : 10000 and i : 20000. 



Like in all other biological reactions, control tests, here two in number, 

 are of the utmost importance. One tube must contain o.i c.c. of the precipi- 

 tating serum mixed with i c.c. of saline, another o.i c.c. of the precipitating 

 serum mixed with a heterologous serum in the dilution of i : 200 and i : 1000. 

 Both of these tubes should show absolutely no precipitate after twenty 

 minutes. In this way the specificity of the precipitin is determined; 

 and it must be remembered that it is the quantitative specificity which 

 counts. 



In the process of the determination of the nature of meats, it is especially 

 necessary to ascertain exactly the precipitating titer against bovine and pig's 

 serum possessed by the rabbit's precipitating serum directed against horse's 

 serum. 



When clear solutions are at hand the precipitin reaction is comparatively 

 simple. Frequently, however, the test must be performed with old and dirty 



