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Dun dun. A crime was committed at the elementary school! The evidence: A hammer was found on the ground near the back entrance of the school. A window was broken. Some computer equipment was missing. A small pool of blood was found on the floor near the window. The suspects: The school’s janitor, who had a bloody bandage around his hand. The patient, who was discovered in the emergency room waiting to get stitches on his arm. The homeless man, who was sleeping outside the school's back entrance. The witness, who saw someone running from the school.

Today, the kids used forensic science and ABO blood typing to figure out whodunit! Before we began our forensic investigation, we took a close look at some doughnuts to learn about antigens and antibodies. The doughnuts represented red blood cells and the sprinkles represented antigens on the cell surface. Some of the doughnuts had white sprinkles (“type A” blood cells), some had brown sprinkles (“type B”), some had both white and brown sprinkles (“type AB”), and some had no sprinkles at all (“type O”). If the doughnuts were exposed to foreign sprinkles (i.e., non-native antigens), the “body” would produce blood proteins known as antibodies. When these antibodies bind to foreign antigens, agglutination (clumping) occurs. Soooo, if a “type A” person receives “type B” or “type AB” blood, their anti-B antibodies will bind to the foreign B antigens and the blood will clump. (That’s bad.) A “type O” person can only receive “type O” blood (since they have both anti-A and anti-B antibodies) but they can donate to any other blood type. This is why “type O” blood is known as the “universal donor”. (That’s nice.)

Once we understood the roles of antigens and antibodies (and ate all the doughnuts), the kids examined the evidence from the “crime scene” to figure out who did it. They were given six “blood” samples (milk + food coloring): one from the hammer near the crime scene, four from the possible suspects, and one from the scene of the crime. They identified the blood types of the samples using specific anti-A and anti-B antibody serums (which contained either water or vinegar). If a blood sample “agglutinated” (i.e., curdled) after adding anti-A serum, then that blood contained “type A” antigens. So who was the evil mastermind behind the crime? Was it the janitor? Was it the eyewitness? Was it the homeless man outside the school? Or was it the injured patient from the hospital? I guess you’ll have to call Poirot… or set up this experiment yourself - it’s really fun!

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