READ: Components of Blood

Introduction

Introduction

 When a stain is found at the scene of a crime, the first thing that has to be determined is whether the stain is blood. Forensic serology is the science that deals with blood and body fluids to determine if it is human and if it can be traced to a particular source. In addition to serological testing, blood spatter analysis can provide valuable information to help reconstruct the events at a crime scene to support or refute statements made by the suspect, the victim, or witnesses.

Essential Questions

1. In what ways does serological evidence help solve a crime?

2. What type of information can be determined from a drop of blood?

3. How is blood typing done?

4. What information can we get from blood stain patterns?

5. How is blood analyzed in the Forensic lab?

6. How is paternity established through blood testing?

Module Minute

Serology involves the properties and reactions of serums, especially blood serum. Often in a crime scene, blood and other body fluids are present and can be tested to identify a suspect or victim if either is unknown. The secrets of the body are written in the blood; a single drop of blood contains red blood cells, white blood cells, platelets and plasma. From the white blood cells, DNA can be extracted and eventually analyzed if enough is present in the sample. Blood stain patterns at a crime scene can help sometimes yield important clues such as the type and velocity of the weapon, the number of blows, the handedness of assailant, position and movements of the victim, order of inflicted wounds, types of injuries, estimation of when crime was committed, and whether the death was immediate or delayed.

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Components of Blood

Components of Blood and Other Serological Evidence

Serology is the medical science dealing with serums found in body fluids such as blood, saliva, sweat, etc. A forensic serologist is a scientist who examines physical evidence in an effort to identify the source of the serum. For the purpose of this class we will focus on blood serum. Blood serum is the yellow fluid that separates from the solid components when whole blood is centrifuged. Blood serum is synonymous with plasma.

Structure and Function of Blood

The primary function of blood is to transport gases such as oxygen and carbon dioxide, distribute nutrients such as glucose, and provide defenses for the body such as platelets for blood clotting and anti-bodies to fight off bacteria, viruses and foreign substances introduced into the blood. Whole blood has a solid component and a liquid component. The solid portion of the blood is composed of red blood cells, white blood cells, and platelets.

Red blood cells (RBCs) are also called erythrocytes. The function of RBCs is to carry oxygen from the lungs to your body's tissues and take carbon dioxide back to your lungs to be exhaled. The surface of RBC's have chemical structures called antigens attached to them. Antigens are substances that stimulate the immune system to manufacture antibodies (immunoglobulins). Antibodies are any of various proteins produced in the blood in response to the presence of an antigen. The function of antibodies is to fight off intruder cells, such as bacteria or viruses, in the body. By becoming attached to antigens on infectious organisms, antibodies can render them harmless or cause them to be destroyed. Antigens stimulate the blood to fight other blood cells that have the wrong antigens. If a person with blood type A is given a transfusion with blood type B, the A antigens will fight the foreign blood cells as though they were an infection.

White blood cells (leukocytes), or WBCs, are one of the body's defenses against disease. Some white cells travel throughout the body and destroy bacteria, some produce antibodies against bacteria and viruses, and others help fight malignant diseases. DNA is gathered from the WBCs in blood samples.

Platelets, or thrombocytes, are small, colorless cell fragments in the blood whose main function is to interact with clotting proteins to stop or prevent bleeding. Interestingly, about 80% of the population are "secretors". This means that the majority of people secrete blood proteins from the blood into other bodily fluids. This is important because it means that the blood type of the person can be determined from a sample of a body fluid besides blood, such as saliva!

 The liquid portion of blood, or plasma, is a fluid, composed of about 92% water; 7% vital proteins such as albumin, gamma globulin, antihemophilic factor, and other clotting factors; and 1% mineral salts, sugars, fats, hormones and vitamins. The primary function of plasma is to serve as the medium that helps blood cells move effectively through the circulatory system. Some of the proteins floating in the plasma are antibodies.

Saliva

The presence of saliva, even if it has dried, can be detected by testing for an enzyme in saliva called amylase. Amylase is an enzyme that breaks down starch and helps the body begin to break down food. Amylase can be found in other body fluids such as blood, so detecting it in a suspected saliva sample is not considered confirmatory for saliva. Saliva has other forensic significance because it can be tested to determine the presence of trace amounts drugs in a body.

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Immunoassay Techniques

Immunoassay Techniques

When a stain is encountered at a crime scene that appears to be blood, there are several questions that must be answered:

1. Is it blood?

2. If it is blood, is it human or some other species?

3. If it is human, can it be linked to a particular individual?

Is it Blood?

 A presumptive, or screening, test for blood often used in crime scenes is the spraying of suspected areas of blood staining with Luminol. If blood was present at some point, the luminol will react with the blood stain and cause the area that was covered with blood to fluoresce. Luminol is simply a chemical reaction in which the Luminol solution reacts with iron from the hemoglobin of blood to cause a faint blue fluorescence. Even tiny amounts of iron can cause Luminol to fluoresce, so it is quite reliable in detecting trace amounts of blood. Because Luminol is extremely sensitive, it can detect even diluted samples of blood that were cleaned up long before. If luminol is sprayed in an area suspected of being a cleaned up crime scene, it will still detect blood that has been diluted up to 300,000 times. If the stain is blood, luminol will not interfere with any subsequent DNA analysis that may be done though it may destroy other components for testing within the blood.

 Another presumptive test is called the Kastle-Meyer Color Test. The Kastle-Meyer Color Test uses a solution of phenolphthalein and hydrogen peroxide on a piece of filter paper to detect blood. When blood of any quantity is present and exposed to the testing paper or solution, it will turn pink.

If it is blood, is it human or some other species?

When trying to determine if a blood stain is human or other animal in origin, it is important to know that various species produce different and distinct proteins and antibodies in the blood and body. Precipitin is an antibody that reacts with its corresponding antigen to form a precipitate if the blood is from a human. Therefore, the precipitin test is done to distinguish human blood from other animal blood. The basic principle of the Precipitin test is that a serum with anti-human antibodies is poured onto or mixed with a blood sample. If the blood sample clots, then the blood sample must be from a human. This type of serum is known as an "Anti-serum" and is created by injecting human blood into an animal, such as a rabbit, to allow the animal's blood to develop antibodies against the human blood. The antibodies are then extracted from the animal's blood serum and placed into the testing serum. This procedure can also be done to produce specific testing serums for all species of animals in addition to the human testing serum. If the blood is identified as human, DNA evidence may be able to link the blood to a specific individual. Blood evidence can also be tested for the presence of drugs or alcohol to yield more information about the person from whom the blood originated.

Georgia Virtual, Forensic Serology, CC BY-NC-SA 3.0