Forensic Blood Analysis - To Whom Does The Blood Belong?

When a human blood stain is found at the scene of a crime, forensic scientists must ascertain the blood's type in a procedure called blood typing. Blood typing refers to the classification of blood based on the presence of inherited antigenic substances on the surfaces of red blood cells (RBC's). Agglutination, or clumping, of RBC's is a positive indicator if the blood is liquid and the RBC's remain intact.

What if the blood is dry? What then?

Most blood found at crime scenes is dry. This kind of blood is most likely going to be a stain that is clotted and dried, and the RBC's have been destroyed. Therefore, clumping of RBC's is highly unlikely, and blood typing can not be conducted. What does the CSI do? He circumvents this problem using a procedure that draws out the remaining antigens.

The absorption-elution procedure is a method that draws out antigens. This is a four-step process:

Treat the object containing the blood stain with antiserums.--The antibodies within the antiserums form a complex with the antigens.

Wash the object.--This steps involves the removal of any excess antiserum-containing antibodies.

Elute the sample.--Eluting breaks down hydrogen bonds between the antigens and antibodies in the presence of heat. The free antibodies are then washed off.

Test the eluted antibodies against known blood antigens.--The antigens to which the antibodies react determine which antigens are present in the original unknown sample of blood.

By simply typing the blood found at a crime scene, forensic scientists can narrow their list of suspects and completely rule out other suspects.

The next time your watch your favorite CSI TV program, you will be more cognizant of the procedures that crime lab technicians use to type, or classify, dried blood.

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