TECHNIC OF PRECIPITIN REACTIONS 303 



of rabbits with certain organ extracts. Thus it is possible to differen- 

 tiate between the liver and kidney of the same animals; such tests have, 

 however, but limited practical value. Maragliano attempted to apply 

 this test of organic specificity to the serodiagnosis of malignant tumors 

 by preparing immune serums by the injection of tumor juices, securing a 

 serum that yielded a precipitate with the albumins of a cancerous tumor. 

 The test is not absolutely specific, and its practical value hi diagnosis 

 requires confirmation. 



TECHNIC OF PREOPITIN REACTIONS 

 DIFFERENTIATION OF HUMAN AND ANIMAL BLOOD BIOLOGIC BLOOD TEST 



Unless a stain is definitely known to be a blood-stain it is necessary 

 to establish its identity by making a chemical test before proceeding with 

 the precipitin reactions. For example, old stains upon clothing may be 

 due to substances other than blood, such as coffee and fruit-juices. 

 Blood-stains upon clothing, metal, wood, or glass may be used for making 

 these reactions and their source determined. 



To identify the stain as one of blood, a portion may be taken into 

 solution in distilled water, rendered slightly acid with dilute acetic acid, 

 filtered until clear, and examined spectroscopically. Or the Teichmann 

 hemin crystal test may be applied to the stain by transferring to a clean 

 slide a small amount of material scraped from the stain; add a few small 

 crystals of sodium chlorid, crush the crystals, and mix the powder with 

 the dry material. Place a clean cover-glass over the stained material 

 and run a small amount of glacial acetic acid under the cover-glass. 

 Heat the preparation to just about the boil '-point for a minute, re- 

 plenishing the acid as may be necessary. The fluid turns brown. 

 The specimen is allowed to cool a few minutes, and is then examined 

 microscopically for the presence of brown rhombic crystals of hemin 

 (Fig. 86). It may be necessary to reheat the specimen several times 

 before the crystals are obtained. 



Having shown that a given stain is actually a blood-stain, the source 

 of the blood may be determined as the result of the precipitin reaction, 

 which consists in extracting the stain in normal salt solution and mixing 

 with antiserums prepared by immunizing rabbits with human and various 

 animal serums. Since the antiserums are known, a precipitate with any 

 one of the extracts indicates that the blood in the stain was derived from 

 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. 



