TECHNIC OF PRECIPITIN REACTIONS 327 



the same species of animal. The reaction is based upon the principle of 

 the specificity of antigen and its antibody. Here the antibody is known, 

 and is used in the test to detect the antigen. 



As mentioned elsewhere, because of the presence of group precipitins 

 the reaction is fraught with certain technical difficulties of importance, 

 especially in medicolegal cases. In most instances it may suffice to show 

 that a stain is of human blood, as will be indicated by a strong reaction 

 with human blood and negative reactions with the bloods of lower ani- 

 mals. If the reaction is negative with antihuman serum, the antiserums 

 of the domestic animals, such as that of the dog, cat, hog, ox, horse, etc., 

 are tried. Although the blood of the higher apes, and even of the lower 

 orders of monkeys, may react slightly with human blood, this factor may 

 be determined by observing a proper technic of dilution, or the possibil- 

 ity of a given stain being one of monkey blood being definitely worked out. 



The reaction can be obtained from blood in an advanced state of 

 putrefaction, or from a stain that has been dried for a year or more. 

 Tests with mummies, however, have reacted negatively, and stains or 

 clots altered by heat may not react unless the antiserum has been pre- 

 pared with a similar antigen. 



This same technic may be employed for the recognition of seminal 

 stains, especially in cases of rape. The stain is taken into solution in 

 exactly the same manner as the blood-stain, and tested with an anti- 

 human semen serum prepared by immunizing rabbits with human semen. 



Preparation of the Extract of Stain (the Precipitinogen). If the 

 stain is on clothing, a portion three inches square should be carefully 

 torn into shreds with forceps and scissors, and not with the fingers, 

 and placed in 40 c.c. of normal salt solution. If the stain is upon wood, 

 glass, or metal, the staining substance should be carefully scraped off 

 with a knife and placed in the salt solution. As a further control on the 

 technic an unstained portion of the clothing should be extracted in the 

 same manner, in order to show that the latter alone does not give the 

 reaction. The mixtures must not be shaken, but should be stood aside 

 for from two to twenty-four hours, depending upon the rapidity of ex- 

 traction. The extract should preferably not be stronger than 1 : 1000. 

 The strength may be estimated by removing 1 c.c. of the extract into 

 another tube, diluting with from 10 to 20 c.c. of normal salt solution, and 

 gently shaking. If a persistent froth appears upon the surface of the 

 fluid, sufficient extraction has occurred. Place 2 c.c. in a test-tube, heat 

 to boiling, and add a drop of a 25 per cent, solution of nitric acid. A 

 faint opalescence indicates that the strength of the extract is about 1 : 1000 

 (Fig. 90). If a heavy precipitate forms, the amount of normal salt solu- 



