LESSON XV 

 FORENSIC PRECIPITIN TEST 



THE precipitin reaction is used in medicolegal work to identify 

 blood stains and to detect adulterations or substitutions in 

 meat or other food products. Since the specificity of the precipi- 

 tin reaction is subject to limitations similar (though not as 

 marked) to those discussed in the case of agglutination, the pre- 

 cipitating serum used in forensic tests must be very potent. 

 Preliminary test should show that the serum gives distinct cloud- 

 ing with its homologous antigen diluted 1 to 1000, within five or 

 ten minutes at room temperature. 



An effort is made to get the suspected protein into approxi- 

 mately this dilution. 



1 



Each student gets : 



2.0 c.c. antihuman serum. 



2.0 c.c. antisheep serum. 



0.4 c.c. normal rabbit serum. 



Two specimens of material with dry blood to be identified. (To some 

 of the students sheep blood is given ; to others human blood or that of 

 other species.) 



From each of the specimens upon which blood has been dried the 

 blood must be soaked off in salt solution. This can be done by placing 

 the material in a test tube with 5 c.c. salt solution and shaking for 

 10 or 15 minutes until the solution presents a lasting foam. This is 

 evidence that the protein is in solution in a concentration of at least 1 part 

 in 1000. Old specimens must often be left in salt solution in the refrig- 

 erator for 24 hours or longer. Such prolonged soaking is done in the 

 refrigerator in order to avoid destruction of the antigen by bacterial 

 growth. Since the nature of the reaction makes it necessary that the 

 material to be examined shall be absolutely clear, it is now cleared by 

 filtration or centrifugation ; if there is enough material the acetic-acid- 

 and-boiling test can be done and the cloud compared in heaviness with 

 a 1-1000 dilution of serum of the suspected variety of blood, similarly 



