How do forensic investigators tell the difference between human and animal blood?

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In order to test blood to see if it is of human origin, a sample of the  blood is collected, and is mixed with anti-human serum. If the blood coagulates, the test is considered positive for human blood.

Anti-human serum is produced by injecting rabbits or other lab animals with serum (also known as plasma, the liquid fraction of the blood) of human blood type O negative. The animals immune systems will react to the foreign non-rabbit proteins present in the human plasma, and they will develop antibodies to these proteins. After the antibodies have formed, blood is drawn from the animal, and the antibodies are extracted from the blood. When these antibodies come in contact with human serum proteins, a clumping or clotting reaction occurs, which can be seen with the naked eye.

If the blood is determined to be human additional tests will be performed to determine blood type, Rh group, and other information.

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