CHAPTER 8 PHYSICAL EVIDENCE: BLOOD
Is blood class evidence or individual evidence? What is the function of blood? What information can be determined from a sample of blood?
Plasma Mainly water, also proteins, hormones, antibodies, & nutrients Red blood cells (erythrocytes) Carry O 2 and CO 2 Hemoglobin=protein that contains iron binds to O 2 & makes blood red
White blood cells (leudocytes) Fight bacteria, viruses, etc Only blood cell with a nucleus Source of DNA Platelets Help with blood clotting Help repair damaged blood vessels
Body can discriminate between self and non-self White blood cells recognize foreign invaders Concentrate in location of invasion Engulf and digest intruder Secrete antibodies proteins that assist in immune response
1982 Dr. Alec Jeffreys produced first DNA profile
Less expensive than DNA profiling Class evidence Can link or exclude a suspect Does not prove guilt Karl Landsteiner developed system for classifying blood types in 1901
Blood type is based on the presence or absence of glycoproteins called antigens Proteins (with sugars attached) found in cell membrane of red blood cells Two types of antigens are A, B
Discovered in 1940 Alexander Weiner worked with Rhesus monkeys Rh + = Blood has Rh factor Rh = Blood does not have Rh factor
Type A (42%) A protein Anti-B antibody Type B (12%) B protein Anti-A antibody
Type AB (3%) Both A & B proteins No antibodies Type O (43%) No A or B proteins Anti-A and Anti-B antibodies
Proteins secreted by B-lymphocytes (white blood cells) Binds to specific antigens Fits like a puzzle piece Recognizes foreign invader and attaches to it Signals white blood cell to destroy foreign invader
If wrong blood type is given, antibodies attack RBC Causes agglutination which is clumping of RBC More than 300 blood group proteins Blood must always be typed and cross checked before transfusions
Used to identify and match blood samples 3 separate tests are done Mix blood with antibodies that bind to each protein (A, B, or Rh factor) Agglutination = protein is present Example Agglutination with protein B and Rh factor = B+
Blood Type? Blood Type? B- A+
To determine probability of two separate events, you multiply their individual probabilities See pg 201 for US population percentages What percentage would have A+ blood? 42% x 85% =.42x.85=.357 = 35.7% What percentage has Type O-, MN? The more blood type proteins you test for, the more you narrow down the suspect pool
A group of bloodstains created when a wound is inflicted 1939 First analysis of blood spatter Blood spatter patterns can give information about Origin of the blood Direction of blood Angle of impact Cause of injury
As blood leaves the body it falls with gravity Cohesion Blood components stick together as blood falls Remains round when it lands If a drop falls on a flat surface, it will have a curved surface.
Natural cohesiveness of blood 22 Forensic Science: Fundamentals & Investigations, Chapter 8
Satellite droplets When blood falls from a height, or at a high velocity, It overcomes its natural cohesiveness, and separates from the main droplet Spiking patterns Form around the droplet edges when blood falls onto a less-than-smooth surface 23 Forensic Science: Fundamentals & Investigations, Chapter 8
Smoother surface = smoother sphere of blood Hard, smooth, non-porous surface Example: glass, tile little or no distortion around edges
Rough or porous surface Example: concrete, carpet Greater distortion and satellites around the edges
Passive Fall Blood falls as a result of gravity Falls at 90-degree angle to floor Circular drops Secondary satellites if surface is textured Examples: Blood dripping from a wound or weapon
Arterial gushes or spurts caused by pumping action of heart through damaged artery such as carotid artery in neck Found on walls or ceiling
Splashes exclamation point pattern that can tell the position of the victim and direction of blood
Smears or transfer blood When a bloody person comes into contact with another surface
Trails Trail of blood drops or spurts
Pools of blood forms around victim who is bleeding heavily in one spot
Round droplets formed by 90-degree drips Droplets with tails or satellite droplets identify direction of origin Void (empty space) could help determine presence of a person or object moved after the attack
Cohesion force of attraction between two similar substances Adhesion attraction between two different substances (blood and wall) Point of impact appears darker and wider Momentum causes tail to form in direction of movement Satellites will appear in front of droplet
Direction of Blood
Draw a line through the long axis of a group of blood stains to determine where they would intersect 2-D model
Use of 2 or more spatter patterns Determine angle of impact 3-D model
Cast-off stains Blood thrown off a blood covered object Usually occurs with repeated blows
Caused by a strong, explosive force Example: gunshot, sneezing Creates a misting of blood Drops less than 1mm
Drops 1-4 mm Beating or stabbing
Drops greater than 4mm Blunt object impact Bleeding after initial injury Walking around after being stabbed or punched
If stain is found Is it blood? Is it human? Blood type DNA
Luminol Used to see blood that may have been cleaned Luminol will react with hydrogen peroxide Iron in hemoglobin will speed up reaction Causes mixture to glow for 30 seconds Is very sensitive so can pick up small amounts of blood
Use a presumptive blood test to determine if evidence is blood Kastle Meyer Test Uses hydrogen peroxide and Phenolphthalein to react to the hemoglobin in blood If sample turns pink then positive for blood Leukomalachite green Turns green in the presence of blood
ELISA test Enzyme linked immunosorbent assay Antibody-protein reaction Human blood is injected into a rabbit Rabbit produces antibodies against human blood Antibodies are tested against blood sample If a reaction occurs then the sample is human
Interpret the findings See if the blood type matches a suspect If it doesn t, excludes the suspect If it does, decide if DNA profiling is needed