TECHNIC OF PRECIPITIN REACTIONS 



309 



In tests other than those employed in medicolegal work, especially 

 if the antiserums are weaker than desired, the reaction may be read after 

 one to two hours. 



In the foregoing test, if positive results are obtained in tubes 1, 2, or 



FIG. 91. A PRECIPITIN REACTION BIOLOGIC BLOOD TEST. 



This reaction was set up with an extract of a stain of sheep blood on a towel, 

 over three months of age, with an anti-sheep immune serum. 



The first tube, containing a 1 : 1000 dilution of the blood (extreme left), shows a 

 well-marked precipitate; in the second (1 : 2000) the reaction is also well marked; 

 the third (1 : 5000) shows no precipitate; the fourth contains a 1 : 1000 dilution of 

 blood-stain, with normal rabbit serum as a control, and shows no precipitate; the 

 fifth is the positive control with known sheep blood extract and immune serum; the 

 last tube is No. 8 of the series, and contains an extract of the non-bloody portion of 

 the towel with immune serum; no precipitate. 



3 and tube 5, and all the others react negatively, the presence of the blood 

 or protein of the species suspected in the unknown extract is established. 

 If the entire test proves negative, the species to which the unknown 

 specimen belongs must be determined with new antiserums prepared for 

 each species, and the tests conducted in the manner described. 



