Chapter 14 Forensic Serology CHAPTER OVERVIEW • Serology involves a broad scope of laboratory tests that use specific antigen and serum antibody reactions. • 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. • 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. • 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. • 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? • The determination of blood is best made by means of a preliminary color test. • A positive result from the Kastle-Meyer color test is highly indicative of blood. • Either the luminol or Bluestar test is used to search out trace amounts of blood located at crime scenes. • 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. • 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. • The best way to locate and characterize a seminal stain is to perform the acid phosphatase color test. • Forensic scientists can successfully link seminal material to an individual by DNA typing. • 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. • 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?