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Forensic Blood Testing - Determining If Blood Is Human

Whenever the presence of blood is confirmed at the scene of a crime, the medical examiner (ME) must ascertain whether the blood is really from a human being and not from a wolf or a bear. After he makes that determination, he can then conduct subsequent testing to determine the person to whom the blood belongs.


The tests used for determining which species that blood belongs to are called antigen-antibody reactions. These are the same tests used for determining blood type in an individual. The main difference is that an antiserum has to be created so that it reacts with antigens that are unique to humans. Antiserum refers to a substance that has antibodies that interact with a specific antigen. An antigen is a molecule that generates an immune response. Therefore, a specific antibody that interacts with a human antigen is created so that the resulting reaction will determine the blood's origin whether it is human or not.


A human antiserum is made by injecting human antigen such as human blood into laboratory rat or any other animal and then allowing enough time for the animal to make enough antibodies specific for the human blood antigen. The animal's blood now containing high quantities of antihuman antibodies is harvested. The antiserum is now isolated for use in testing blood samples to find out whether they are from humans.


If a substance containing antiserum interacts with a blood sample containing the antigen, a chemical reaction occurs. The reaction makes an antigen-antibody complex that forms a precipitate. The effect is a visible precipitation line falling out where the two reagents, the antiserum and antigen, meet.


Put in other ways, if an antibody against the human blood antigen comes into contact with a substance containing human blood, the reaction produces a visible line of precipitation, or solid substance between the two solutions. If there is no chemical reaction that produces a precipitate, then the ME can conclude that the blood is not human.


The modern day forensics laboratory contains various antiserums of common animal bloods. Sheep, cow, deer, wolf, and cat antiserums are commercially available. Using these antiserums, a criminalist can determine what species shed the blood.


The next time you watch your favorite CSI program, you will be better cognizant of the chemicals used in the crime lab to determine if a blood stain they find is really human.


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