Chapter 14
Forensic Serology
CHAPTER OVERVIEW
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Serology involves a broad scope of laboratory tests that use specific antigen and serum
antibody reactions.
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An antibody reacts or agglutinates only with its specific antigen. The concept of specific
antigen–antibody reactions has been applied to techniques for the detection of drugs of
abuse in blood and urine.
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When an animal is injected with an antigen, its body produces 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.
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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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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 with a particular individual?
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The determination of blood is best made by means of a preliminary color test.
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A positive result from the Kastle-Meyer color test is highly indicative of blood.
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Either the luminol or Bluestar test is used to search out trace amounts of blood located at
crime scenes.
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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.
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At one time, 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 newer DNA technology.
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The best way to locate and characterize a seminal stain is to perform the acid phosphatase
color test.
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Forensic scientists can successfully link seminal material to an individual by DNA
typing.
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A rape victim must undergo a medical examination as soon as possible after the assault.
At that time clothing, hairs, and vaginal and rectal swabs can be collected for subsequent
laboratory examination.
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If a suspect is apprehended within 24 hours of a sexual assault, it may be possible to
detect the victim’s DNA on the suspect’s underwear or on a penile swab of the suspect.
Questions
1.
What is the A-B-O system? Why is the system no longer used by forensic scientists?
2.
What is an antigen and how is it useful in individualizing blood?
3.
What is an antibody, and what happens when an antibody reacts with its specific antigen?
4.
What factor is most whole blood typed for? What is the most common blood type in the
United States? Which is least common?
5.
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?
6.
What is the difference between monoclonal and polyclonal antibodies? Which type are
more useful for the forensic scientist and why?
7.
What three questions must the criminalist be prepared to answer when examining dried
blood?
8.
List two commonly used color tests for blood. How does a luminol test differ from these
tests?
9.
What is the purpose of a precipitin test? Name three strengths of precipitin tests.
10.
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?
11.
What is acid phosphatase and how is it used by forensic scientists?
12.
List three reasons why spermatozoa are often not found in seminal fluid collected at a
crime scene.
13.
Why is it important for investigators to seek information about when and if voluntary
sexual activity last occurred before a sexual assault?