Forensic Serology: Blood and Blood Spatter Evidence

The Composition of Blood Plasma – the fluid portion of unclotted 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 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 and no other. 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 TypeAntigens on Red Blood Cells Antibodies in Serum AAAnti-B BBAnti – A AB Neither OnoneBoth

Rhesus Factor Represented by + and - but are NOT positive and negative. A person either has the rhesus factor or they don’t. Eighty-five percent of the human population has the Rh factor.

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?

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 spatter patterns 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 fallSplashesTrails Arterial spurtsSmearsPools

Examining Directionality There are several forces that affect 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 VelocitySize of DropletsVisual ImageVelocity of Blood Examples of Injuries HighLess the 1 mm100 ft/secGunshot wounds Medium1-4 mm25 ft/secBeating, stabbing Low4-6 mm5 ft/secBlunt object impact