Carnegie Mellon University


City Hospital reported that an unconscious and bleeding man was found lying just outside the emergency room door. Staff members said that the ER was unusually quiet that night and many of them heard a car leaving the entrance area at a high rate of speed. When they looked outside, they found the injured man. They believe that he was dropped off by someone in the car. No one saw the car.

When the man regained consciousness, he reported that he had gotten into an argument with his neighbor, and that they began to fight. He remembers that his neighbor picked up a garden tool. The next thing he remembers is waking up in the hospital about 30 minutes later.

Police talked to the neighbor, who denied being involved in a fight. He said that the injured man was falsely accusing him because the man believes that he was the one who turned the man's son into the police for vandalizing cars in the neighborhood.

Police examined the neighbor’s car and found traces of blood.  The neighbor said he had cut his hand at work recently and that the blood was his.  Police recovered the blood from the car and also obtained blood samples from the injured man and from the neighbor.

Determine the blood type of all three samples to see whose blood was in the car.


Blood is composed of living cells in a complex mixture called serum. The most abundant of these cells is the red blood cell.  It contains iron which gives it its red color, and it transports oxygen and carbon dioxide through the body.

Attached to the exterior of the red blood cell are proteins called ANTIGENS. Antigens are produced in the cell by its genes. The gene may produce one of two types of antigen or none at all. These two types of antigens are designated A and B.  People are assigned a blood type based on which antigen is found on their red blood cells.  Those with antigen A are in the Type A blood group and those with antigen B are in the B group. Those whose blood cells contain no antigens are placed in a group called Type O. Some people have blood cells which produce both antigen A and antigen B.  Their blood is classified as Type AB. In the United States, approximately 45% of the population have Type O blood, 41% have Type A, 10% have Type 8, and only 4% have Type AB.

Blood serum also contains other proteins that help the body fight disease. These proteins are known as ANTIBODIES. Antibodies are produced by the immune system and attack  specific "foreign" materials that enter the blood stream, such as viruses, by attaching themselves to the "invader''. Viruses with antibodies attached are easily destroyed by the white cells in the blood. Sometimes antibody-coated cells and viruses may clump together, in an action known as AGGLUTINATION.

There are also two types of antibodies, anti-A and anti-B. Blood serum contains antibodies against A or B antigens if they are not present on its red blood cells.  A person with type A blood would have A antigens on his cells and anti-B antibodies in his serum. The anti-B antibodies would attack red blood cells with type B antigen but would not attack those with A antigens. A person with type B blood would have anti-A antibodies which would attack type A antigens but not its own B antigens.  A person with Type O blood, who has neither A or B antigens, has both anti-A and anti-B antibodies, and a person with type AB blood which contains both A and B antigens has no anti-A or anti-B antibodies. Table 1 summarizes this information.


Blood Type Antigen Type Antibody Type
A A Anti-B
B B Anti-A
O NONE Anti-A and Anti-B

Blood typing information is critical during blood transfusions.  If a person with type A blood is given type B blood, the anti-B antibodies in their blood will attach to the B antigens on the type B blood cells and cause clumping (agglutination) which can be fatal as small blood vessels become clogged and blood flow is stopped.

The process of agglutination and the information on antigens and antibodies in Table 1 is the basis of blood typing.  A drop of blood is mixed with a drop of liquid that contains anti-A antibodies and another is mixed with liquid that contains anti-B antibodies.  The mixtures are then examined for agglutination. Table 2 summarizes the results of the tests for each blood type.  In a positive test (+), blood agglutination or clumping is observed, while in a negative test (-), no agglutination is observed.


Blood Type Anti-A Antibody Test Anti-B Antibody Test
A + -
B - +
AB + +
O - -

As you can see from Table 2, the anti-A antibody will attack the A antigen in type A or AB blood.  The anti-B antibody will attack the B antigen in Type B or AB blood.  Neither antibody will attack Type O blood which contains neither A or B antigens.

Blood typing is useful to forensic scientists because it can show the possible match of two samples.  More importantly, it can show that two samples do not match and could have not had the same origin. This result is known as "exclusion" and is a very important concept in criminal investigations. If the samples do not match, then the suspect is "excluded" as the culprit.  If there is a match, he is "included" as only a possible suspect.  Only DNA testing can identify the origin of a blood sample to a 99.95% certainty.

We will test simulated blood with simulated antibodies and check for a positive or negative result with each to determine the blood type. No real blood or blood products will be used in any of the tests. The test materials cannot be used to type real human blood, however the actual typing of human blood is done in a similar way.