Forensic Serology Blood and Blood Spatter Evidence The
Forensic Serology: Blood and Blood Spatter Evidence
The Composition of Blood • Plasma – the fluid portion of un-clotted blood. • Red blood cells (erythrocytes) - donut shaped cells that carry oxygen throughout the body. • White blood cells (leukocytes) - cells that destroy foreign materials. • Platelets – aid in blood clotting and repair of damaged blood vessels.
Antigens and Antibodies • Red blood cells have a surface that contains chemical structures called antigens. – Antigen – a substance, usually a protein, that stimulates the body to produce antibodies against it. • Of the more than 15 blood antigen systems (blood types) identified, the A-B-O and Rh are the most important.
– Antibody – a protein in the blood serum that destroys or inactivates a specific antigen. – Antiserum – blood serum that contains specific antibodies. – Agglutination – the clumping together of red blood cells by the action of an antibody. – An antibody reacts only with its specific antigen. The antibodies can simultaneously attach to antigens on two different red blood cells. This creates agglutination.
Normal Red Cells Agglutinated red cells
Blood Typing • Serology – the study of antigen-antibody reactions. Blood Type Antigens on Red Blood Cells Antibodies in Serum A A Anti-B B B Anti – A AB AB Neither O none Both
Rhesus Factor • Represented by Rh+ and Rh– A person either has the rhesus factor (Rh+) or they don’t (Rh-). • 85% of the human population Rh+.
Testing for the Presence of Blood • There are three questions that have to be answered when looking at possible blood stains/spatter. 1. Is it blood? 2. Is it human blood? 3. If it is human, can it link to an individual?
Presumptive vs Confirmatory • A presumptive test - a qualitative analysis that allows to identify, or confirm, the presence of a substance in a sample. **Could be blood • A confirmatory test - proves that the substance is actually human blood.
• Kastle-Meyer color test – tests for the presence of hemoglobin (red blood cell protein) that has a peroxidase-like activity. • Luminol – reacts to blood to emit light and thus requires the result to be observed in a darkened area. • Precipitin Test – an antibody that reacts with its corresponding antigen to form a precipitate to distinguish animal from human blood.
Blood Spatter • Blood spatterns can be used to explain events at a violent crime scene by examining the location, distribution and appearance of the spatters. • Observations and conclusions that can be used by investigators to determine the direction, dropping distance (height), and angle of impact can include:
1. Surface texture – harder/softer and porous/nonporous 2. Direction of travel – shape 3. Impact angle 4. Origin – area of convergence
Blood Spatter Classifications Passive fall Splashes Trails Arterial spurts Smears Pools
Examining Directionality • There are several forces that affect a droplet of blood that is moving. • Cohesion – a force between two similar substances. • Adhesion – a force between two unlike surfaces, such as blood and the surface of a wall. • Surface tension – an elastic characteristic along the outer edge of a liquid caused by the attraction of like molecules.
Impact Angle
Area of Convergence
Point of Origin
Blood Spatter Types Velocity Size of Droplets Visual Image Velocity of Blood Examples of Injuries High Less the 1 mm 100 ft/sec Gunshot wounds Medium 1 -4 mm 25 ft/sec Beating, stabbing Low 4 -6 mm 5 ft/sec Blunt object impact
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