Blood

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 7,000–25,000 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 (1868–1943), 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 molecules—protein antigens A and B—on 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.