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Chapter 8 Blood & Blood Splatter By the end of this chapter you will be able to:
• explain the composition of blood • describe the function of blood cells • determine the blood type of a blood sample • conduct a blood splatter analysis • examine wounds and describe the nature of the weapon • find and process blood evidence
Introduction
• Blood typing is class evidence; whereas DNA profiling (using white blood cells) is individual evidence. • A blood splatter pattern can give information about the truthfulness of an account by a witness or a suspect.• It can provide info about the direction from which the blood originated, the point of origin, the angle and velocity of impact, and type of weapon used. • Our understanding of blood began in ancient times and continues to grow today.
History
• 2500B.C.: Egyptian bloodlettingeffort to cure disease• 500B.C.: Greeks discovered difference between arteries and veins• 1659: Leeuwenhoek saw blood cells w/ microscope• 19011902: blood types discovered• 1940: Rh protein discovered• 1959: Congofirst recorded case of AIDS• 1984: HIV identified as cause of AIDS
Composition of Blood
• Blood is a circulating tissue• Whole blood carries cells and plasma—the fluid with hormones, antibodies, clotting factors, and nutrients.
plasma is similar to salt water in composition • Red blood cells (erythrocytes) carry oxygen to the body’s cells and carbon dioxide away.
Hemoglobin binds & transports O2, makes blood red• White blood cells (leukocytes) fight disease and foreign invaders and are only ones that contain nuclei & DNA. • Platelets aid in blood clotting and the repair of damaged blood vessels.
Blood Typing—Proteins
42% 12% 3% 43% of the population in the United States (of which 85% is Rh+)
• Blood typing is quicker and less expensive than DNA profiling. • It produces class evidence but can still link a suspect to a crime scene or exclude a suspect.
Discovery of Blood types
• 1900 Landsteiner found that blood from one person did not always mix with blood from another
clumping might occur which could result in death• The presence or absence of particular cellsurface proteins found embedded within the cell or plasma membranes of red blood cells determine a person's blood type• An antibody reaction test is used to identify each blood type
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A and B Proteins
• If a person's blood contains only protein A, then he/she has type A blood (same is true for type B)• If a person has neither A or B proteins, then he/she has type O blood. If person has both A and B, then he/she has type AB blood.
Genotype Blood TypeAA or AO ABB or BO B
AB ABOO O
Rh Factor
• In 1940, Weiner worked with Rhesus monkeys and discovered another protein on red blood cells• 85% of humans have the Rh protein (factor) and are designated as Rh+• Blood types are typically defined by the presence or absence of the A, B and Rh proteins.• When a person receives a blood protein that is foreign to him or her, antibodies will cause the blood to clump and may cause death.
Blood Typing—Antibodies; Additional Proteins and Enzymes
• Blymphocytes are specialized white blood cells that secrete antibodies• Antibodies proteins secreted by white blood cells that attach to specific antigens to destroy them. • Antigens foreign molecules or cells that react to antibodies. • Antigenantibody response White blood cells recognize a substance as foreign and try to destroy it• B lymphocytes makes antibodies against the foreign blood protein which attach to the foreign blood cells• Phagocytes,a type of white blood cell, engulfs it
AB
AB+
AB
A+
A
B+
B
O+
O
Blood Typing Agglutination
• Agglutination clumping of molecules or cells caused by an antigenantibody response• Each red blood cell has more than 1 million protein binding sites and there are more than 300 known blood group proteins, such as M and N proteins.• Antibodies are Yshaped and each arm of the Y attaches to a different red blood cell when it recognizes a foreign protein causing the cells to clump together• If clumping occurs within a person, the blood flow is blocked
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Blood Typing Tests
• Blood is tested for A, B and Rh proteins• Three tests are performed.
Blood is mixed with antibodies that bind to each of the three proteins.
If blood clumps (agglutinates) when mixed with the antibodies for protein A, that means the person's blood contains protein A.
B and Rh are tested the same way People with type AB+ would have clumps with all
three tests. O would not clump with any test
Blood Typing—Probability and Blood Types
• Since we know frequency of different genes, it is possible to determine the probability that a particular blood type will appear in a particular population. • To determine the probability of two separate events, we multiply their individual probabilities. • By identifying the additional proteins in the blood evidence sample, investigators can limit the size of a suspect population and help identify a suspect.
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