Blood (Encyclopedia of Science)
Blood is a fluid connective tissue that performs many functions in the body. It carries oxygen and nutrients to the cells, hormones (chemical messengers) to the tissues, and waste products to organs that remove them from the body. Blood also acts as a defense against foreign microorganisms and helps to keep the body at a constant temperature in warm-blooded animals.
Blood consists of white blood cells, red blood cells, and platelets suspended in plasma, a watery, straw-colored fluid. Plasma makes up about 55 percent of the blood, while blood cells and platelets make up the remaining 45 percent. The average adult human body contains about 6 quarts (approximately 5.6 microliters) of blood.
Plasma is made up of 92 percent water, 7 percent proteins, salts, and other substances it transports. Fibrinogen is an important protein involved in blood clotting. Albumins and globulins are proteins that aid in the regulation of fluid in and out of the blood vessels. Proteins called gamma globulins act as antibodies and help protect the body against foreign substances, called antigens.
The salts present in plasma include sodium, potassium, calcium, magnesium, chloride, and bicarbonate. They are involved in many important body functions such as muscle contraction, the transmission of nerve impulses, and regulation of the body's...
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Blood (Encyclopedia of Nursing & Allied Health)
Blood is a liquid connective tissue that performs many functions in the body, including transport of oxygen, carbon dioxide, nutrients, waste products, and hormones; clotting; and defense against microorganisms. Blood consists blood cells suspended in plasma, a fluid that contains proteins, salts, and other substances. When a blood sample is placed in a test tube and spun rapidly (a process called centrifugation), the heavier blood cells sink to the bottom of the test tube, while the straw-colored plasma floats on top.
All vertebrates circulate blood within blood vessels. Because blood is enclosed within blood vessels, the circulatory systems of vertebrates are called closed circulatory systems. (Some invertebrates have open circulatory systems that do not contain blood vessels and circulate a blood-like fluid called hemolymph.)
The human body contains about 4 to 6.3 qt (4 to 6 L) of blood. Men have more blood than women, due to the presence of higher levels of testosterone, a hormone that regulates sex characteristics and function and also stimulates red blood cell formation. Plasma makes up 55% of the blood, while the blood cells constitute the other 45%. The various types of blood cells are red blood cells (erythrocytes), white blood cells (leukocytes or leucocytes), and platelets.
Plasma contains mostly water, which accounts for91.5% of the plasma content. The water acts as a solvent for carrying other substances.
PLASMA PROTEINS. Proteins account for 7% of plasma. The higher concentration of protein in blood prevents water from moving from the blood into the interstitial fluid. Without this osmotic protection, water would move from the blood into the interstitial fluid, causing a rapid loss of blood volume.
The most abundant of the plasma proteins is albumin, a protein also found in egg white. Albumin concentration is four times higher in the blood than in the interstitial fluid (the watery fluid that bathes tissues, but is located outside and between cells). This high concentration of albumin in plasma serves an important osmotic function.
Other proteins that are present in plasma are immunoglobins and fibrinogen. Immunoglobins, also called antibodies, are proteins that function in the immune response. Antibodies attach to invading bacteria and other microorganisms, marking them for destruction by immune cells. Fibrinogen is a protein that functions in a complex series of reactions that leads to the formation of blood clots.
OTHER PLASMA COMPONENTS. The other components of plasma are salts, nutrients, enzymes, hormones, and nitrogenous waste products. Together, these substances account for 1.5% of plasma. The salts present in plasma include sodium, potassium, calcium, magnesium, chloride, and bicarbonate. These salts function in many important body processes. For instance, calcium functions in muscle contraction; sodium, chloride, and potassium function in nerve impulse transmission in nerve cells; and bicarbonate regulates pH. These salts are also called electrolytes. An imbalance of electrolytes, which can be caused by dehydration, can be a serious medical condition. Many gastrointestinal illnesses, such as cholera, cause a loss of electrolytes through severe diarrhea. When electrolytes are lost, they must be replaced with intravenous or oral solutions of water and salts.
The remaining substances present in plasma are elements that the plasma is transporting from one place to another. For instance, plasma contains nutrients that nourish tissues. The nutrients found in plasma include amino acids, the building blocks of proteins; glucose and other sugars; and fatty acids and glycerol, the components of lipids (fats). In addition to nutrients, plasma also contains enzymes, or small proteins that function in chemical reactions, and hormones, which are transported from glands to body tissues. Waste products from the breakdown of proteins are also found in plasma. These waste products include creatinine, uric acid, and ammonium salts. Blood transports these waste products from the body tissues to the kidneys, where they are filtered from the blood and excreted in the urine.
Red blood cells
The human body contains an estimated 25 trillion red blood cells; approximately 4.8 million to 5.4 million are found in every microliter of blood. The structure of a red blood cell is eminently suited to its primary function, the transport of oxygen from the lungs to body tissues. Red blood cells are very small (about 6 nanometers wide), shaped like a disk, and contain a small depression on either side. Their small size allows them to squeeze through the tiniest of blood vessels (capillaries). In addition, their size allows a greater diffusion of oxygen across the blood cells' plasma membranes than if the cells were largerecause blood contains so many of these small cells, their combined surface areas translate into an extremely large surface area for the diffusion of oxygen. The disk shape and the depressions on either side also contribute to a greater surface area.
TRANSPORT OF OXYGEN. Red blood cells are unusual in that they do not contain nuclei or mitochondria, the cellular organelles in which aerobic metabolism (the breakdown of nutrients that requires oxygen) is carried out. Instead, red blood cells acquire energy through metabolic processes that do not require oxygen. The lack of nuclei and mitochondria therefore allow the red blood cell to function without depleting its cargo of oxygen, leaving more oxygen for the body tissues.
The molecule that binds oxygen in red blood cells is called hemoglobin. Hemoglobin is a large, globular protein consisting of four protein chains surrounding an iron core. Hemoglobin is densely packed inside the red blood cell; in fact, hemoglobin accounts for a third of the weight of the entire red blood cell. Each red blood cell contains about 250 molecules of hemoglobin.
In the lungs, oxygen diffuses across the red blood cell membrane and binds to hemoglobin. As blood circulates to the tissues, oxygen diffuses out of the red blood cells and enters tissues. The waste product of aerobic metabolism, carbon dioxide, then diffuses across red blood cells and binds to hemoglobin. Once circulated back to the lungs, the red blood cells discharge their load of carbon dioxide, which is then breathed out of the lungs. However, only 7% of carbon dioxide generated from metabolism is transported back to the lungs for exhalation by red blood cells; the majority is transported in the form of bicarbonate, a component of plasma.
HEMOPOIESIS. Red blood cells are formed in red bone marrow from precursor cells called pluripotent stem cells. The process of red blood cell formation is called hemopoiesis (alternatively, hematopoiesis). In adults hemopoiesis takes place in the marrow of ribs, vertebrae, the breastbone, and the pelvis. On average, a red blood cell lives only three to four months. Constant wear and tear on the red blood cell membrane, caused by squeezing through tiny capillaries, contributes to the red blood cell's short life span. Worn out red blood cells are destroyed by phagocytic cells (cells that engulf and digest other cells) in the liver. Parts of red blood cells are recycled for use in other red blood cells, such as the iron component of hemoglobin.
White blood cells
White blood cells are less numerous than red blood cells in the human body; each microliter of blood contains 5,000 to 10,000 white blood cells. The number of white blood cells increases, however, when the body is fighting off infection. Their numbers are maintained until the immune system detects the presence of a foreign invader. When the immune system is activated, chemicals called lymphokines stimulate the production of more white blood cells.
White blood cells function in the body's defense against invasion and are key components of the immune system. They usually do not circulate in the blood vessels, and are instead found in the interstitial fluid and in lymph nodes. Lymph nodes are composed of lymphatic tissue and are located at strategic places in the body. Blood filters through the lymph nodes, and the white cells present in the nodes attack and destroy any foreign invaders.
TYPES OF WHITE BLOOD CELLS. The human body contains five types of white blood cells: monocytes, neutrophils, basophils, eosinophils, and lymphocytes. Each type of white blood cell plays a specific role in the body's immune defense system.
Under a microscope, three kinds of white blood cells appear to contain granules within their cytoplasm. These three types are the neutrophils, basophils, and eosinophils. Together, these three types of white blood cells are called granulocytes. The granules are specific chemicals that are released during the immune response. The other two types of white blood cells, the monocytes and lymphocytes, do not contain granules. These types are known as the agranular leukocytes.
Monocytes, which comprise 3% to 8% of the white blood cells, and neutrophils, which comprise 60% to 70% of white blood cells, are called phagocytes. They ingest and digest cells, including foreign microorganisms such as bacteria. Monocytes differentiate into cells called macrophages. Macrophages can be fixed in one place, such as in the brain and lymph nodes, or can "wander" to areas where they are needed, such as the site of an infection. Neutrophils have an additional defensive property: they release granules of lysozyme, an enzyme that destroys cells.
Basophils comprise 0.5% to 1% of the total composition of white blood cells and function in the body's inflammatory response. Allergies are caused by an inflammatory response to relatively harmless substances, such as pollen or dust, in sensitive individuals. When activated, basophils release various chemicals that cause the characteristic symptoms of allergies. Histamines, for instance, cause the runny nose and watery eyes associated with allergic reactions; heparin is an anticoagulant that slows blood clotting and encourages the flow of blood to the site of inflammation, inducing swelling.
Eosinophils, which comprise 2% to 4% of the total composition of white blood cells, are believed to counteract the effects of histamine and other inflammatory chemicals. They also phagocytize bacteria tagged by antibodies.
Lymphocytes, which comprise 20% to 25% of the total composition of white blood cells, are divided into two types: B lymphocytes (also called B cells) and T lymphocytes (also called T cells). The names of these lymphocytes are derived from their origin. T lymphocytes are named for the thymus, an organ located in the upper chest region where these cells mature; and B lymphocytes are named for the bursa of Fabricus, an organ in birds where these cells were discovered.
T lymphocytes play key roles in the immune response. One type of T lymphocyte, the helper T lymphocyte, activates the immune response when it encounters a macrophage that has ingested a foreign microorganism. Another kind of T lymphocyte, called a cytotoxic T lymphocyte, kills cells infected by foreign microorganisms. B lymphocytes, when activated by helper T lymphocytes, become plasma cells, which in turn secrete large amounts of antibodies.
All white blood cells arise in the red bone marrow. However, the cells destined to become lymphocytes are first differentiated into lymphoid stem cells in the red bone marrow. These stem cells undergo further development and maturation in the spleen, tonsils, thymus, adenoids, and lymph nodes.
Platelets are not cells; they are fragments of cells that function in blood clotting. Platelets number about 250,000 to 400,000 per liter of blood. Blood clotting is a complex process that involves a cascade of reactions that leads to the formation of a blood clot. Platelets contain chemicals called clotting factors. These clotting factors first combine with a protein called prothrombin. This reaction converts prothrombin to thrombin. Thrombin, in turn, converts fibrinogen (present in plasma) to fibrin. Fibrin is a thread-like protein that traps red blood cells as they leak out of a cut in the skin. As the clot hardens, it forms a seal over the cut.
This process works for relatively small cuts in the skin. When a cut is large, or if an artery is severed, blood loss is so severe that the physical pressure of the blood leaving the body prevents clots from forming. In addition, in the inherited disorder called hemophilia, one or more clotting factors are lacking in the platelets. This disorder causes severe bleeding from even the most minor cuts and bruises.
Platelets have a short life span; they survive for only five to nine days before being replaced. Platelets are produced in red bone marrow and are broken off from other red blood cells.
Role in human health
Researchers hope to create synthetic blood substitutes to ease the burden of dwindling blood donations that are needed to meet the demand for surgeries, transfusions, and emergencies. Currently under development are blood substitutes that use perfluorocarbons or modified hemoglobin to carry oxygen to tissues.
Perfluorocarbons are long, fatty hydrocarbon chains containing fluorine that have the ability to pick up oxygen in lungs and release it into tissues. The artificial blood is a mixture of perfluorocarbons with saline (physiological salt water) using surfactants, substances that allow the mixing of oil and water. The solution then can be administered to patients. Over time, as the artificial blood helps deliver oxygen to tissues, the perflourocarbon molecules are exhaled from the body.
Hemoglobin solutions contain hemoglobin that has been isolated from red blood cells and chemically altered to increase its lifespan in the bloodstream and to ensure adequate oxygen-carrying capabilities.
Strictly, these substances are not whole blood substitutes since they only have the ability to carry oxygen and cannot replace the other important functions of blood. However, they would be valuable in eliminating the risk of transmitting disease during transfusions as well as preventing accidental blood type mismatches.
ABO BLOOD GROUPS. An interesting aspect of red blood cells is that they carry certain proteins, called antigens, on their plasma membranes. These antigens are responsible for the various blood groups known as A, B, AB, and O:
- A person with A antigens is type A and has antibodies to B antigens.
- A person with B antigens is type B and has antibodies to A antigens.
- A person with both antigens is type AB and does not have antibodies to either antigen.
- A person with none of the antigens is type O and has antibodies to both A and B antigens.
These combinations are necessary to know when performing a blood transfusion. For instance, if a type A individual donates blood to a type B individual, the A antibodies in the recipient's B blood will react with the A antigens of the donor's A blood. This reaction, called the agglutination reaction, causes the blood cells to clump together. Agglutination can be fatal. Until blood typing was worked out early in this century, many deaths from blood transfusions occurred due to incompatibility of antigens and antibodies.
HLA ANTIGEN GROUPS. Like red blood cells, the plasma membranes of white blood cells also contain antigens. These surface antigens are called the human leukocyte associated (HLA) antigens. Like the red blood cell types, these HLA antigens represent different white blood cell "groups." When a person receives an organ transplanted from a donor, the recipient and the donor must have the same HLA antigen group for the transplant to be successful. If the donor and recipient are two different HLA antigen groups, the recipient's body will "reject" the organ; in other words, the recipient's immune system will be activated by the foreign cells of the organ and initiate an immune response against the organ.
Common diseases and disorders
Sickle cell anemia
Sickle cell anemia is an inherited disorder caused by a defect in one of hemoglobin's four protein chains. The defective hemoglobin distorts the shape of the red blood cells and injuries the red blood cell membrane. Water and potassium leak from the cells, causing the red blood cells to become rigid and "sickle-shaped." As a result of these changes, oxygen transport is severely interrupted and circulation of the blood through the blood vessels can become blocked. These irregular blood cells do not carry as much oxygen as their normally shaped counterparts. Although the prognosis for individuals with sickle cell anemia was historically poor, improvements in life expectancy and quality have been made due to early diagnosis and treatment.
Hemophilia is hereditary group of bleeding disorders that results in insufficient clotting and excessive bleeding. Types are hemophilia A, hemophilia B, and von Willebrand's disease. Hemophilia A is the most common type. It results from a deficiency in clotting factor VIII. Only males have this sex-linked disease, but women may be carriers. Uncontrolled bleeding, both internal and
external, may be caused by the smallest of injuries. Treatment involves clotting factor supplementation, and tranfusions are common when blood is lost, or prophylactically.
Human immunodeficiency virus
Human immunodeficiency virus (HIV), the causative agent of acquired immune deficiency syndrome (AIDS), attacks and kills T lymphocytes. This dis-ease cripples the immune system and leaves the body helpless to stave off infections. As AIDS progresses, the number of helper T lymphocytes drops from a normal 1,000 per cubic millimeter to below 200.
Aerobic metabolismetabolic processes that require oxygen.
Antibodyn immune protein that marks foreign microorganisms in the body for destruction by other immune cells.
Antigen protein that is attached to a cell's plasma membrane.
Centrifugation laboratory procedure in which a test tube of blood or other liquid is spun at a high speed.
Clotting factor set of substances released by platelets that function in the clotting mechanism.
Electrolyteshe salts and other substances present in the plasma that function in crucial body processes.
Fibrin protein that functions in the clotting mechanism; forms mesh-like threads that trap red blood cells.
Fibrinogenhe inactive form of fibrin present in plasma; activated by clotting factors released by platelets.
Hemoglobinhe protein found in red blood cells that binds oxygen; consists of four protein chains surrounding an iron core.
Hemophilia genetic disorder in which one or more clotting factors are not released by the platelets; causes severe bleeding from even minor cuts and bruises.
Hemopoiesishe process of red blood cell formation in the bone marrow.
Histamine chemical released by basophils during the inflammatory response; causes blood vessels to dilate.
Inflammatory response type of non-specific immune response; involves the release of chemicals from basophils that increase blood circulation and white blood cell migration to the affected area.
Interstitial fluidhe fluid that bathes cells.
Lymph node small structure located at several points in the body; consists of lymphatic tissue that filters blood and removes microorganisms.
Lymphocyte type of white blood cell; includes B and T lymphocytes.
Lysozymen enzyme released by neutrophils that kills cells.
Lymphoid stem cellhe cell from which B and T lymphocytes are derived.
Phagocytizeo engulf and digest a cell.
Plasma cellhe cell derived from the B lymphocyte, which secretes antibodies.
Pluripotent stem cellhe type of stem cell from which red blood cells and more white blood cells are derived in the bone marrow.
Sickle cell anemia genetic disorder caused by a defect in one of hemoglobin's four protein chains; causes red blood cells to be sickle-shaped.
Long, Michael W. and Max S. Wicha, eds. The Hematopoietic Microenvironment: The Functional and Structural Basis of Blood Cell Development. Baltimore: Johns Hopkins University Press, 1993.
Shin, Linda, and Karen Belliner. Blood and Coagulation Disorders Sourcebook: Basic Information about Blood and Its Components. Omnigraphics, 1998.
Creteur, Jacques, William Sibbald, and Jean-Louis Vincent. "Hemoglobin Solutions: Not just Red Blood Cell Substitutes." Critical Care Medicine 28 (August 2000): 3025-34.
Delves, P. J. and I. M. Roitt. "The Immune System: First of Two Parts." New England Journal of Medicine 343 (6 July 2000): 37-49.
Delves, P. J. and I. M. Roitt. "The Immune System: Second of Two Parts." New England Journal of Medicine 343 (13 July 2000): 108-17.
Tremper, Kevin K. "Perfluorochemical Blood Substitutes: Indications for an Oxygen-Carrying Colloid." Anesthesiology 91 (November 1999): 1185.
American Sickle Cell Anemia Association. 10300 Carnegie Avenue, East Office Building (EEb18), Cleveland, OH44106. (216) 229-8600. <<a href="http://www.ascaa.org">http://www.ascaa.org>.
America's Blood Centers. 725 15th Street NW, Suite 700, Washington, DC 20005. (202) 393-5725. <<a href="http://www.americasblood.org">http://www.americasblood.org>.
National Hemophilia Foundation. 116 West 32nd Street, 11th Floor, New York, NY 10001. (800) 42-HANDI. <<a href="http://www.hemophilia.org">http://www.hemophilia.org>.
"Blood Work: A Useful Tool for Monitoring HIV." Project Inform Website. April 2001. 7 July 2001. <<a href="http://www.projinf.org/fs/HIVDiagTest.html">http://www.projinf.org/fs/HIVDiagTest.html>.
"Sickle Cell Anemia." KidsHealth Website. 2001. 7 July 2001. <<a href="http://kidshealth.org/parent/medical/heart/sickle_cell_anemia.html">http://kidshealth.org/parent/medical/heart/sickle_cell_anem... >.
Blood (World of Forensic Science)
Frequently, the forensic analysis of a crime or accident scene will involve the analysis of blood. Whether in the form of fresh liquid, dried blood, jelly-like coagulated blood, or patchy drops or stains, blood can be a treasure trove of information. As one example, the pattern of a bloodstain can tell a forensic investigator much about the nature of the accident or crime. Just as important is the composition of the blood.
A typical human body contains approximately ten pints (4.7 liters) of blood. Depending on the severity of a wound, blood can be lost slowly or, as in the case of a severed artery, can spurt quickly out of the body. A forensic examiner can tell a great deal about the nature of the accident or crime from the pattern of the blood residue. Additionally, knowledge of the composition of blood and properties of these components is also valuable in identifying a victim or implicating an assailant.
Human blood is made up of several different types of cells. Each has a distinctive appearance and function.
Red blood cells are absolutely vital for life. Each drop of blood contains millions of these cells. In the body, the circulating red blood cells deliver oxygen to cells and transport waste material from the cells.
Red blood cells are round, smooth-edged, and saucer-like in shape, typically having a slightly depressed center. In a disease like anemia or sickle cell anemia, the cells can be present in reduced numbers or can adopt an abnormal sickle shape. This reduces the oxygen carrying capacity of the blood. The presence of such abnormalities can alert a forensic investigator or medical examiner to the presence of disease or poison, or lack of constituents, including iron, vitamin B12, or folic acid, or other maladies.
The bright red color of a healthy red blood cell comes from the presence of an iron-containing compound called hemoglobin. The presence of iron makes hemoglobin an excellent molecule for the binding and transport of oxygen and carbon dioxide. As blood passes through the tiny channels that permeate the lung, the oxygen molecules that diffuse across the channel membrane bind to the hemoglobin. The oxygen is subsequently released to cells all through the body during the circulation of the red blood cells.
Once vacant, the binding site in the hemoglobin is able to accommodate the binding of carbon dioxides and other waste products of cellular metabolism. These products, which would become toxic to the cells if allowed to accumulate, are then transported away. As the red blood cells pass back through the lung, the carbon dioxide and other waste molecules are released from the hemoglobin and are exhaled.
Red blood cells are long-lived, but not immortal. The average lifetime is approximately 120 days. Although cells are continually dying and being replenished, the number of red blood cells remains constant in a properly-operating body.
In contrast to the smooth, plate-like red blood cells, white blood cells are spheres that have numerous knob-like projections sticking out from their surface.
White blood cells are part of the body's defense system against infection. When a microbial threat is recognized by the immune system, white blood cells are signaled and directed to the site of the threat. There, they attack the invading microorganisms, by producing antibodies directed against components of the microbe or by physically engulfing, ingesting, and dissolving the invader.
White blood cells are primed and ready for their defensive duties by means of a short life span. They live only a few days to several weeks.
Under normal conditions there are 7,0005,000 white blood cells per drop of blood. The determination of this number can provide an indication of the presence of disease. For example, if a bacterial, viral, or parasitic infection proves resistant to eradication, an increased number of white blood cells will be recruited to do battle with the invader, reducing the white blood cell count in the blood. Conversely, cancer of the blood (leukemia) causes the numbers of white blood cells to increase markedly. A leukemia patient can display upwards of 50,000 white blood cells per drop of blood.
The bloodstain that confronts a forensic investigator at the site of an accident or crime may be the result of a catastrophic injury that the body was unable to repair. Normally, the cuts and scrapes that occur during the normal course of life can be addressed by sealing up the wound.
The patching of a wound is the task of the colorless blood cells called platelets. Platelets do not have a uniform shape. Rather, they are reminiscent of an amoeba, being blob-like, with long and thin surface projections.
Platelets are recruited to the site of a cut or wound. Their shape and sticky surface facilitates their clumping together, along with calcium, vitamin K, and a protein called fibrinogen. The clump is known as a clot.
Clot formation is a complicated process that involves a cascade of biochemical reactions. Without platelets, clotting would not occur. When in the vicinity of the open wound, and so in the presence of an increased concentration of oxygen, the platelets dissolve. A consequence of the dissolution is the conversion of fibrinogen to fibrin. The tiny thread-like fibrin molecules collect to form a mesh that entraps intact and dissolved blood cells and other constituents. As this mass hardens, the clot forms. A hardened clot is also called a scab.
This effective wound patching system does have its limits, however. In the case of a catastrophic injury such as a knife or bullet wound, bleeding may continue unabated. If not treated, such a wound can be fatal.
The various blood cells are suspended in a straw-colored liquid called plasma. Plasma is composed mainly of water. Physiologically-important ions including calcium, sodium, potassium and magnesium also comprise plasma.
Plasma provides the medium in which the blood cells are suspended and transported around the body. As well, the disease-fighting antibodies produced by the immune system are also ferried to where they are needed via the plasma.
Blood, specifically the red blood cells, are also a valuable resource for a forensic investigator, as the cells can be used to determine what is known as the blood type of the victim or assailant.
The chemical residues present on the surface of red blood cells are the basis of blood typing. These were first described early in the twentieth century by the Austrian-born American immunologist Karl Landsteiner (1868943), who subsequently developed the typing criteria. For his achievements, Landsteiner was awarded the 1930 Nobel Prize in Medicine.
Landsteiner noted the presence of two distinct moleculesrotein antigens A and Bn the surface of red blood cells. Type A blood is comprised of red blood cells that have only the A molecule, whereas the red blood cells of type B blood have only the B molecule. The presence of both molecules occurs in type AB blood. Finally, red blood cells can be devoid of both molecules. This occurs in type O blood.
The determination of blood type can be easily done by mixing a sample of blood with antibodies to the A or B components. In the presence of the correct antibody, the blood cells will clump together, forming a visible precipitate.
Blood typing remains a powerful forensic tool in linking someone to the crime or accident scene. In addition, because blood type is a genetically acquired trait, blood typing can be useful in establishing familial relationships. However, because a great many people have the same blood type, this test alone is not a definitive identification.
Another very useful aspect of blood in forensic examinations involves a factor known as the Rh (for Rhesus) factor. The factor, which was also discovered by Landsteiner, derives its name from the Rhesus monkey, a species similar to us and so one that is used in medical studies. The Rh factor of human blood was discovered in blood comparisons between humans and the Rhesus monkey.
Rh factor is a protein that is present in the blood of some people (who are described as Rh positive, or Rh+. Some people lack the blood protein, and so are described as being Rh negative (Rh-).
The determination of the Rh status of a blood sample provides another piece of evidence that can help determine the identify of the victim or link someone to the crime or accident.
In addition to the A and B antigens and Rh factor, modern day blood typing includes over 150 blood-borne proteins and 250 enzymes located in blood cells.
This extensive form of blood typing, while still useful, is laborious and has been largely replaced by the molecular precision of genetic analysis.
As with every other cell in the body, blood cells contain genetic material in the form of deoxyribonucleic acid (DNA). DNA can be isolated and subjected to a variety of sophisticated analyses to determine the sequence of the nucleotide building blocks that comprise the structure. As well, small sequences that tend to vary from person to person can be quickly copied over and over again, using the polymerase chain reaction (PCR), to produce sufficient quantities for the sequence analysis. In this way, the pattern of DNA that is unique to an individual can be revealed.
Recovering the same DNA pattern in a blood sample of a suspect and from blood recovered at a crime scene is very powerful evidence tying the person to the crime scene. As seen in the trial of O.J. Simpson, however, even this evidence can fail to sway a jury if not convincingly presented or defended.
SEE ALSO Blood spatter; Bloodstain evidence; Blood volume test; Blood, presumptive test; Cast-off blood; DNA; Toxins; Wound assessment.