Transcript Document

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Chapter 10
FORENSIC SEROLOGY
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Nature of Blood
• The word blood refers to a highly complex mixture
of cells, enzymes, proteins, and inorganic
substances.
• Plasma, which is the fluid portion of blood, is
composed principally of water.
• Red blood cells (erythrocytes), white blood cells
(leukocytes), and platelets are the solid materials
suspended in plasma.
• Antigens, usually proteins, are located on the
surface of red blood cells and are responsible for
blood-type characteristics.
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Blood Typing
• More than 15 blood antigen systems have been identified, but the
A-B-O and Rh systems are the most important.
• An individual that is type A has A antigens on his/her red blood
cells, type B has B antigens, AB has both A and B antigens, and
type O has neither A nor B antigens.
• Rh factor is determined by the presence of another antigen, the D
antigen.
• People having the D antigen are Rh positive; those not having the
antigen are Rh negative.
• For every antigen there is a specific antibody that will react with it
to form clumps known as agglutination.
• Thus, if serum containing anti-B is added to red blood cells
carrying B antigen, they will immediately react.
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Serology
• The term serology is used to describe a broad
scope of laboratory tests that use specific
antigen and serum antibody reactions.
• The identity of each of the four A-B-O blood
groups can be established by testing the blood
with anti-A and anti-B sera.
• The concept of specific antigen-antibody
reactions has been applied to immunoassay
techniques for the detection of drugs of abuse in
blood and urine.
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Immunoassay
• A number of immunological assay techniques are
commercially available for detecting drugs through
antigen-antibody reaction.
• One such technique, the enzyme-multiplied
immunoassay technique (EMIT), is used by
toxicologists because of its speed and high sensitivity
for detecting drugs in urine.
• In a typical EMIT analysis, antibodies that will bind to
a specific drug are added to the subject’s urine.
• Other immunoassay procedures are also available,
such as radioimmunoassay (RIA) which uses drugs
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labeled with radioactive tags.
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Antigen-Antibody
Reaction
• When an animal, such as a rabbit or mouse, is
injected with an antigen its body will produce a
series of different antibodies, all of which are
designed to attack some particular site on the
antigen of interest.
• This collection of antibodies is known as
polyclonal antibodies.
• Alternately, a more uniform and specific collection
of antibodies designed to combine with a single
antigen site can be manufactured.
• Such antibodies are known as monoclonals.
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Forensics of Blood
• The criminalist must be prepared to answer the
following questions when examining dried blood:
• (1) Is it blood?
• (2) From what species did the blood originate?
• (3) If the blood is of human origin, how closely can it be
associated to a particular individual?
• The determination of blood is best made by means of
a preliminary color test.
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The Tests
• A positive result from the Kastle-Meyer color test
is highly indicative of blood.
• Hemoglobin causes a deep pink color.
• Alternatively, the luminol test is used to search out
trace amounts of blood located at crime scenes.
• Produces light (luminescence) in a darkened area.
• Microcrystalline tests, such as the Takayama and
Teichmann tests, depend on the addition of specific
chemicals to the blood so that characteristic
crystals will be formed.
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The Tests
• Once the stain has been characterized as blood,
the precipitin test will determine whether the
stain is of human or animal origin.
• The precipitin test uses antisera normally
derived from rabbits that have been injected with
the blood of a known animal to determine the
species origin of a questioned bloodstain.
• Once it has been determined that the bloodstain
is of human origin, an effort must be made to
associate or dissociate the stain with a particular
individual.
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A-B-O vs DNA
• Prior to the advent of DNA typing, bloodstains were
linked to a source by A-B-O typing and the
characterization of polymorphic blood enzymes and
proteins.
• This approach has now been supplanted by the newer
DNA technology.
• DNA analysis has allowed forensic scientists to
associate blood and semen stains to a single individual.
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Stain Patterns of
Blood
• The crime scene investigator must remember
that the location, distribution, and appearance
of bloodstains and spatters may be useful for
interpreting and reconstructing the events that
produced the bleeding.
• Surface texture and the stain’s shape, size, and
location must be considered when determining
the direction, dropping distance, and angle of
impact of a bloodstain.
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Stain Patterns of
Blood
• Surface texture is of paramount importance. In general, the
harder and less porous the surface, the less spatter results.
• The direction of travel of blood striking an object may be
discerned because the pointed end of a bloodstain always faces
its direction of travel.
• The impact angle of blood on a flat surface can be determined
by measuring the degree of circular distortion. At right angles
the blood drop is circular, as the angle decreases, the stain
becomes elongated.
• The origin of a blood spatter in a two-dimensional configuration
can be established by drawing straight lines through the long
axis of several individual bloodstains. The intersection or point
of convergence of the lines represents the origin point.
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Heredity and
Paternity
• The transmission of hereditary material is
accomplished by means of microscopic units
called genes, located on chromosomes.
• Alternative forms of genes that influence a given
characteristic (such as eye color or blood type)
are known as alleles.
• Paternity testing has historically involved the
A-B-O blood typing system, along with blood
factors other than A-B-O.
• Currently, paternity testing has implemented
DNA test procedures that can raise the odds of
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establishing paternity beyond
99 percent.
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Testing for Seminal
Stains
• Many of the cases sent to a forensic laboratory involve
sexual offenses, making it necessary to examine
exhibits for the presence of seminal stains.
• The best way to locate and at the same time
characterize a seminal stain is to perform the acid
phosphatase (an enzyme secreted into seminal fluid)
color test.
• A purple color indicates acid phosphatase enzyme.
• Semen can be unequivocally identified by either the
presence of spermatozoa or of p30, a protein unique
to seminal plasma.
• Forensic scientists can successfully link seminal
material to an individual by DNA typing.
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Rape Evidence
• The rape victim must undergo a medical
examination as soon as possible after the assault.
• At that time the appropriate items of physical
evidence including clothing, hairs, and vaginal and
rectal swabs can be collected for subsequent
laboratory examination.
• All outer and undergarments should be carefully
removed and packaged separately in paper (not
plastic) bags.
• Bedding, or the object upon which the assault took
place, may also be carefully collected.
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Rape Evidence
• If a suspect is apprehended within 24 hours of
the assault, it may be possible to detect the
victim’s DNA on the male’s underwear or on a
penile swab of the suspect.
• Items routinely collected from the suspect
include all clothing, pubic hair, head hair, penile
swab, and a blood sample or buccal swab for
DNA typing.
• The forceful physical contact between victim
and assailant may result in a transfer of such
physical evidence of blood, semen, saliva, hairs,
and fibers.
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Figure 10–1
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Figure 10–3
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Figure 10-7
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Figure 10-8
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Figure 12–7 Illustration of stain convergence on a two-dimensional
plane. Convergence represents the point from which the stains
emanated. Courtesy Judith Bunker, J. L. Bunker & Assoc., Ocoee, FL
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Figure 10-12
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Figure 10-13 An antibody–antigen–antibody sandwich or complex is
seen as a colored band. This signifies the presence of PSA in the extract
of a stain and positively identifies human semen.
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Review questions 1-5
• What is the A-B-O system? Why is the system no longer used by
forensic scientists?
• What is an antigen and how is it useful in individualizing blood?
• What is an antibody and what happens when an antibody reacts
with its specific antigen?
• What factor is most whole blood typed for? What is the most
common blood type in the United States? Which is least
common?
• For what other application do forensic scientists often use specific
antigen–
– antibody reactions? What is the EMIT technique
frequently used for, and what is its greatest limitation?
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Review Questions 6-10
• What is the difference between monoclonal and polyclonal
antibodies? Which type are more useful for the forensic scientist
and why?
• What three questions must the criminalist be prepared to answer
when examining dried blood?
• List and describe the commonly used color tests for blood. How
does a luminol test differ from these tests?
• What is the purpose of a precipitin test? Describe three strengths
of precipitin tests. What is a genotype and how do parents’
’
genotypes affect the blood type of their offspring? In what area of
the law does this information have important implications?
• What is acid phosphatase and how is it used by forensic scientists?
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Review questions 11-14
• List three reasons why spermatozoa are often not found in
seminal fluid collected at a crime scene.
• Why is it important for investigators to seek information about
when and if voluntary sexual activity last occurred before a sexual
assault?
• A dead rape victim is found in her apartment. Describe the proper
collection and preservation of the seminal stained clothing.
Emphasize the proper collection of controls from the victim and
suspect.
• The victim of a homicide is wrapped in a blood-soaked sheet.
Describe the proper steps to be taken in order to preserve the sheet
for laboratory examination. List all necessary submissions that
must be made for a thorough examination of blood evidence.
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© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
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