FORENSIC LAB ACTIVITY--BLOOD ANALYSIS

Background Information
Blood is a red, sticky fluid. The watery part is called plasma. Several kinds of cells are suspended in the plasma: red blood cells, white blood cells and platelets. Many chemicals are also suspended or dissolved in the plasma, including proteins, sugars, fats, salts, enzymes and gases. Each person's blood has certain inherited characteristics that distinguish it from he blood of other people. But only recently have scientists developed the ability to identify most of these characteristics. Until the 1980's, blood was primarily differentiated according to the presence of three substances that are on the outside of red blood cells called antigens. These are the A, B, and D antigens. The presence or absence of A and B antigens on the the blood determines a persons blood type. If the A antigen is present, the person has A type blood. If the B antigen is present, he has B type blood. If both antigens are present, the blood type is AB. If no antigens are present the person has O type blood. The other important blood antigen is the Rh factor, or D antigen. If one has the D antigen, the person is Rh positive. If he lacks it, he is Rh negative. In order to type a person's blood, antibodies (called agglutinins) made to the antigen are added to the drop of blood. If clumping occurs, then the blood type is known.

anti-A agglutinin + blood
clumping occurs
anti-B agglutinin + blood
clumping occurs
Blood type
yes
no
A
no
yes
B
yes
yes
AB
no
no
O


Procedure: Case 96-3337
(Note: because of blood-borne pathogens, you will be working with simulated, not real blood) Sign and date the label on the evidence bags.

  1. Place 2 drops of blood from the crime scene (Exhibit D) on the microscope slide.

  2. Add a drop of Anti-A antiserum to the drop and stir it with the yellow toothpick. Immediately throw the toothpick into the disposal container.

  3. Add a drop of Anti-B antiserum to the second drop of blood. Stir it with the blue toothpick and immediated throw the toothpick away.

  4. Observe the clumping and determine the blood type.

  5. Repeat the procedure for blood from Exhibit E, F, and G. Be sure to record your data. Place used slides in dishpan.



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