Chapter 10 FORENSIC SEROLOGY - PowerPoint PPT Presentation

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  2. 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. FORENSIC SEROLOGY

  3. 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. FORENSIC SEROLOGY

  4. 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. FORENSIC SEROLOGY

  5. 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 labeled with radioactive tags. FORENSIC SEROLOGY

  6. 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. FORENSIC SEROLOGY

  7. 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. FORENSIC SEROLOGY

  8. 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. FORENSIC SEROLOGY

  9. 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. FORENSIC SEROLOGY

  10. 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. FORENSIC SEROLOGY

  11. 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. FORENSIC SEROLOGY

  12. 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. FORENSIC SEROLOGY

  13. 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 establishing paternity beyond 99 percent. FORENSIC SEROLOGY

  14. 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. FORENSIC SEROLOGY

  15. 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. FORENSIC SEROLOGY

  16. 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. FORENSIC SEROLOGY

  17. Figure 10–1  

  18. Figure 10–3  

  19. Figure 10-7  

  20. Figure 10-8  

  21. 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

  22. Figure 10-12  

  23. 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.

  24. 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?

  25. 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?

  26. 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.