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Notes for the Instructor

Blood is the liquid that flows throughout the circulatory system bringing oxygen to the body. Blood found at a crime scene can tell the story of what happened. Forensic scientists are brought in to study the amount, its location, and the spatter pattern of blood. This can reveal whether or not violence may be associated with the crime, how injured someone is, and what movements the injured person made at the scene. Blood can also be used to tell investigators more about the injured individual.

Human blood is commonly categorized using the A-B-O system. First developed in 1901 by Karl Landsteiner, the A-B-O system distinguishes blood based on which antigens – proteins on the surface of red blood cells – are present or absent in the blood. An antigen triggers an immune response in your body when a foreign object is detected. Antigens in the blood help your body distinguish between its own blood and another type of blood that it should fight against.

People who have type A blood have A antigens. Their bodies will consider any blood with B antigens to be foreign, and attack it. People with type B blood have B antigens. Their bodies will consider any blood with A antigens to be foreign, and attack it.

There are people who have type AB blood, with both A and B antigens. Their bodies will accept any type of blood – A or B – without attacking it. People with type AB blood are called universal recipients.

A fourth group of people have neither A nor B antigens in their blood. These people are considered to have type O blood. A person with type O blood can give their blood to any person, since their blood lacks both A and B antigens. People with type O blood are called universal donors.

Another piece in determining your blood type is your Rhesus factor, or Rh factor. Rh is another protein found on the surface of your red blood cells. People who have Rh are called Rh positive. People who lack Rh are considered Rh negative.

Your blood type is a combination of your ABO factor and your Rh-factor. For example, someone could be AB+ or O-.

Since blood can reveal information about a person, it is helpful to the investigation if it is found at the crime scene. Sometimes blood can be found at the crime scene, even if it is not visible to the naked eye. Luminol is a chemical that glows in the presence of blood. Investigators can spray Luminol around the crime scene to determine if blood is present but invisible. Luminol acts quickly, taking only about five seconds. The room needs to be dark to view the glow. The more blood present, the stronger the glow. Luminol reacts to old blood and areas where blood has been wiped away. The challenge with Luminol is that it can make blood ineffective for further testing.

The Kastle-Meyer test is another method of detecting blood at a crime scene. It uses a solution that turns pink when blood is present. The challenge with the Kastle-Meyer test is that it may also react positively to other substances, not just blood.

If blood is found at the crime scene, investigators may test it to determine the blood type using the A-B-O system. This is one way to learn more about the injured person.

In this lesson, students will simulate blood typing using synthetic blood. The reactions will mirror reactions observed in actual testing. As part of The Cookie Jar Mystery, Mrs. Randall found a small amount of blood on a piece of the broken cookie jar. Students will compare the blood type found on the cookie jar with the blood type of each suspect.

Identifying the blood type of the sample found at the crime scene will provide another piece of class evidence for the investigation. Remember, class evidence is evidence that points to a group – or class – of people. In this case, identifying the blood type would point to all people who share the blood type identified.

The activities in this lesson address Next Generation Science Standards practices of Planning and Carrying Out Investigations and Analyzing and Interpreting Data. In addition, they address Common Core Learning Standards. See the appendix on page 105 for more details.