Top Shelf Science: Forensics BLOOD EVIDENCE

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Approximatelyone-twelfth

of the humanbody is blood.

Blood as Forensic Evidence

What Is Blood!

Approximately one-twelfth of the human, body is blood, which isbasically cells suspendedina liquid. The fluid.portion is calledplasma (55% of the blood). It is composed of 90% wate-r and10% metabolites-waste, salts, ions (mostly Na". cr RCO) ], andproteins. The solid portion of blood is composed of three principaltypes of cells: (1) Red (erythrocytes), which contain hemoglobin,transport oxygen from the lungs to the cells and in return carry.carbon dioxide back to the lungs, where it is exhaled. (2) Whitecells (leukocytes) are the primary cells of the immune system.They produce antibodies. (3) Platelets start the clotting proce.rsby initiating the formation of fibrin to form a clot. Removi.dg thesolid clotting material leaves a pale yellow, watery fluid calledserum.

In 1901, Austrian biologist Karl Landsteiner recognized thatall human blood was not the same and worked out the ABO

classification system to describe the differences. This was importantbecause so many blood transfusions had resulted in immediatedeath of the patient for no apparent reason. In 1940, Landsteinerdiscovered the rhesus factor (Rh) in blood. Now over 100 differentfactors are known to exist. Theoretically, no one, except identicaltwins, has the same combination of blood factors; practically,however, the complete identification is difficult, time-consuming,and expensive. Also, many factors break down as blood dries andages. In the forensic-science world, blood-factor identification isnot yet practical as a means of individualization. DNA analysis, onthe other hand, offers individualization, but also is time-consum-ing and expensive. Nevertheless, ABa and Rh blood characteriza-tion is an important component of forensic serology because it canbe done on whole blood as well as dried blood stains, quickly andwithout expensive apparatus. Additionally, ~bout 80% of thepopulation are secretors, meaning their blood-type antigens arefound in body fluids other than jUst blood.

Present on the surface' of each red blood cell (RBe) are millions ofcharacteristic chemical structures called antigens. These proteinsare responsible for the different blood types. For the ABO system,there are two types of antigens, A and, B. Type A blood cells have

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Top Shelf Science: Forensics BLOOD EVIDENCE Blood as Forensic Evidence

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A antigens, type B blood cells have B antigens, type AB blood cellshave both A and B antigens, and type 0 blood cells have neitherantigen,

Some white blood cells manufacture proteins called antibodies thatare found in the serum, the yellowish liquid that separates fromblood when a clot is formed. These antibodies, known as antiserum,are produced to attack invaders that enter the blood stream (anti-gens that do not belong in ycmr system, such as snake venom, .bacteria, or someone else's blood). For example, when virusesresponsible for the mumps enter the blood, the body recognizesthem as foreign and begins synthesizing antibodies that combineonly with the specific antigens of the virus, Antibody-coatedviruses are destroyed by white blood cells. If the person is againexposed to mumps, the existing antibodies are able to prevent arepetition of the illness. This is the basis of vaccines,

A person with type A blood (A antigens on their red blood cells)will produce specific antibodies in their serum to attack anddestroy type B blood cells as they are introduced into the body.Type & serum containing the B antibodies is called anti-B anti-bodies or anti-B serum, because It destroys type B blood cells.Likewise, a person with type B blood will have A antibodies intheir serum. Type AB blood has both A and B antigens on the redblood cells and no antibodies in the serum. Type 0 blood, whichhas neither A nor B antigens, has anti-A and anti-B antibodiesin its serum. This is confusing-· you may have to think about itfor a while to fully understand. Table 1 provides a summary ofblood types.

TABLE 1 Blood Types

Blood TypeAntigens AntibodiesonRBC in Serum

A A anti-B

B B anti-A

AB AandB none

·0 none anti-A and anti-B

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Top Shelf Science: Forensics BLOOD EVIDENCE Blood as Forensic Evidence

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Figure 3Representation of Agglutination

TABLE3

BloodType

Percentage

Type A 41

TypeB 10

TypeAB 4

Type 0 45

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Antibodies are bivalent, which means that they can attach totwo antigen sites, thus causing agglutination, or clumping,similar to cross-linking in polymers. If a person with type Bblood receives a transfusion of type A blood, the anti-Aantibodies in the recipient's B blood will attach to thedonor's A red blood cells and cause agglutination, asdepicted in Figure 3..

The result can be fataL This is why ABO typing is necessarybefore undergoing blood transfusions. The.anti-B antibodiesreceived from the donor 'are diluted by the larger volumeof the recipient's blood, so the transfused antibodies donot-cause a problem. Table 2 shows safe blood-donor and.recipient combinations.

TABLE2 Safe ABO Combinations

BloodAntibody

Donor RecipientType

AntigenFor For

A A anti-B A,AB A,O

B B anti-A B,AB B;O

AB A&B none AB all

0 none anti-A & all 0anti-B

The Rh factor is another important means of classifying blood typefor forensic serology. It is sometimes referred to as the D antigen.Those who have it are called Rh positive (Rh+); those without it,Rh negative (Rh-). Approximately 85% of the population is Rh+.

The distribution of blood type varies both with location and racethroughout the world. In the United States, a typical ABO distribu-tion is illustrated in Table 3.

Thus, blood typing can be important in forensic science becauseit can show that two samples did not have the same origin; that is,typing evidence can exclude suspects. Also, the distribution ofblood types within a specific population can be used to statisticallydetermine the probability of someone having a particularcombina-tion of blood types. For example, what is the probability of aperson having type B+ blood?

10 85 850 ..- X - = or about lout of 12 people.100 100 10,000 . .

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ToP Shelf Science: Forensics BLOOD EVIDENCE Blood as Forensic Evidence

.- Exploration Activities

1. What is the probability of a person having type AB- blood? Show your calculations.

2. Blood typing can be applied to a host of enzymes and proteins that perform specificfunctions in the body. Their presence or absence varies within the population. Morethan 150 serum proteins and 250 cellular enzymes have been isolated. Therefore, itis possible to use blood typing as individual evidence; however, it is not practicalto achieve the statistics required because of the time and techniques involved. Also,most factors degrade with time. Rather, ABOfRh typing, and often another kind oftyping called MNS, are used as exclusionary tests in forensic science and paternitytesting. The typical population in the United States shows an MNS distribution ofM ::: 30%, N::: 27%, S :::48%. If a blood stain found at the scene of a crime is foundto be B, N, Rh-, what are the chances that a suspect would have this combination ofantigens? Is this good enough to convince a jury?

3. How much blood is there in the average adult human?

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