Chapter 15 Forensic Serology

Nature of Blood*

• Plasma, which is the fluid portion of blood, is composed principally of water and accounts for 55% of blood content.

• Red blood cells (erythrocytes), white blood cells (leukocytes), and platelets are the solid materials suspended in plasma.

• Antigens, usually proteins and carbohydrates, are located on the surface of red blood cells and are responsible for blood-type characteristics.

• Plasma contains proteins known as antibodies. The fundamental principle of blood typing is that for every antigen there exists a specific antibody.

• Antibodies are normally bivalent—that is they have two reactive sites. This means that an antibody can attach to antigens located on two red blood cells forming a network of cross-linked cells seen as clumping or agglutination.

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.

• Approximately 43% of the population is type O, 42% type A, 12% type B, and 3% type AB.

•  Type A blood has only anti-B and no anti-A. Type B blood has only anti-A and no anti-B. Type AB blood has neither anti-A nor anti-B. Type O blood has both anti-A and anti-B.

• 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.*

Immunoassay

• The concept of specific antigen-antibody reactions has been applied to immunoassay techniques for detecting drugs-of- abuse in blood and urine.

•  Antibodies that react with drugs do not exist naturally; however, they can be produced in animals by combining the drug with a protein and injecting this combination into the animal.

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.

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.

• This is followed by adding to the urine a chemically labeled version of the drug. A competition will ensue between the labeled and unlabeled drug to bind with the antibody.

Forensics of Blood

• The criminalist must be prepared to answer the following questions when examining dried blood:

  •  Is it blood?

  •  From what species did the blood originate?

  • 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.

Blood Color 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 or Bluestar test are used to search out trace amounts of blood located at crime scenes.

  • Produces light (luminescence) in a darkened area.

  •  Luminol is extremely sensitive and is capable of detecting blood that has been diluted up to 100,000 times.

Blood Origin Testing

• 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.

• The technique of gel diffusion takes advantage of the fact that antibodies and antigens diffuse or move toward one another on an agar plate.

• The extracted bloodstain and the human antiserum are placed in separate holes opposite each other on the gel. If the blood is human, a line of precipitation forms where the antigens and antibodies meet.

Individualizing Bloodstains

• 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.

• DNA analysis has allowed forensic scientists to associate blood to a single individual.

Genes and Chromosomes

• The gene is the basic unit of heredity. A chromosome is a threadlike structure in the cell nucleus along which the genes are located. Each gene is actually composed of DNA specifically designed to carry the task of controlling the genetic traits of our cells.

• The position a gene occupies on a chromosome is known as a locus. Approximately 30,000 human genes have been identified.

• Most human cells contain 46 chromosomes, arranged in 23 mated pairs. The only exceptions are the human reproductive cells, the egg and sperm, which contain 23 unmated chromosomes.

• During fertilization, a sperm and an egg combine so that each contributes 23 chromosomes to form the new cell, or zygote, that develops into the offspring.

• An allele is any of several alternative forms of genes at a particular locus and that are aligned with one another on a chromosome pair.

• A heterozygous gene pair is made up of two different alleles; a homozygous gene pair is made up of two similar alleles.

• When two different genes are inherited, the characteristic coded for by a dominant gene will be expressed. The characteristic coded for by a recessive gene will remain hidden.

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.

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 specifically identified by the presence of spermatozoa, or tentatively identified by p30, a protein present in seminal plasma.

•  Forensic scientists can successfully link seminal material to an individual by DNA typing.

Sexual Assault Evidence

• The sexual assault victim must undergo a medical examination as soon as possible after the assault.

• At that time the appropriate items of physical evidence are collected:

  • – Pubic Combings

  • – Pubic Hair Controls (25)

  • – External Genital Swabs

  • – Vaginal Swabs

  • – Cervix Swabs

  • – Rectal Swab

  • – Swabs of body areas such as breasts

• At that time the appropriate items of physical evidence are collected:

  • – Oral Swabs

  • – Head hairs (25)

  • – Blood Sample

  • – Buccal Swab

  • – Fingernail Scrapings

  • – Urine Specimen

  • – All Clothing

• 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.

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.

The Persistence of Seminal Constituents in the Vagina

• Motile Sperm

  • – 4 to 6 hours

• Non-motile Sperm

  • 3 to 6 days

•  Seminal Acid Phosphatase

  •  Less than 48 hours

• Prostate Specific Antigen

  • Less than 72 hours

Packaging Biological Evidence 

• Before the collection of biological evidence begins, it is important that it be photographed and recorded on sketches. 

• Wearing disposable latex gloves while handling the evidence is required.

• The packaging of biological evidence in plastic or airtight containers must be avoided because the accumulation of residual moisture could contribute to the growth of DNA- destroying bacteria and fungi

• Each stained article should be packaged separately in a paper bag or in a well-ventilated box.

•  All biological evidence should be refrigerated or stored in a cool location until delivery to the laboratory.