PRECIPITIN REACTIONS 305 



cause the formation of a corresponding precipitate. If this is then 

 removed by centrifugation (quantitative relations being, of course, 

 duly considered) the remaining serum may be shown to have retained 

 its precipitin for human serum, while that for monkey serum has 

 disappeared. That antihuman serum, moreover, should possess a 

 larger quantity of antihuman than of antimonkey precipitin would 

 naturally suggest itself and can be demonstrated by suitable methods. 



The technique which is involved in these various examinations 

 may be suitably described in connection with its application to the 

 medico-legal blood test, according to Uhlenhuth. 



The Biological Blood Test. As the legal question at issue is usually 

 whether or not a certain blood-stain is of human origin, it is ordinarily 

 only necessary to examine the material in question in reference to 

 its behavior toward an antihuman serum. If, on the other hand, 

 the antihuman investigation has shown that the material was not 

 of human origin, and it is desired to ascertain from what animal 

 species the blood was derived, corresponding sera must, of course, 

 also be available. 



PREPARATION OF THE ANTISERA. The antisera in question are 

 usually obtained from rabbits after injection with either human 

 serum, pig serum, or bovine serum, etc., as the case may be. The 

 injections are given intraperitoneally or intravenously at intervals 

 of five or six days, using 10, 8, and 5 c.c. respectively in the first 

 instance, and 5, 3, and 2 c.c. if the latter method is preferred. It is 

 always best to inject several rabbits at the same time, especially since 

 not every animal furnishes a serum with a sufficiently high titer. 

 According to Uhlenhuth this should be such that 0.1 c.c. of the anti- 

 serum shall produce a distinct turbidity either instantaneously or at 

 most after one to two minutes, when added to 1 c.c. of a 1 to 1000 

 dilution of the corresponding antigenic serum. Added to 1 c.c. of a 

 1 to 10,000 and 1 to 20,000 dilution a turbidity should be discernible 

 after three and five minutes respectively, while a control specimen, 

 containing only 0.85 per cent, saline (the diluent in question) and 

 0.1 c.c. of the antiserum must, of course, remain clear. The reaction 

 is best observed by holding the tubes (without shaking) against a 

 dark background, when it will be seen that the turbidity first appears 

 as a faint opalescence at the bottom of the tubes, but in the course 

 of five minutes extends throughout the specimen, becoming increas- 

 ingly denser and ultimately settling to the bottom as a precipitate. 

 20 



