Notes on Forensic Serology

1. Learning Objectives

In this lesson, students will learn how to:

  • Provide comprehensive definitions for all key terms related to forensic serology.

  • List the A-B-O antigens and antibodies found in the blood for each of the four blood types: A, B, AB, and O.

  • Explain the mechanism behind agglutination.

  • Explain the process of whole blood typing.

  • Describe forensic tests used to characterize a stain as blood.

  • Demonstrate an understanding of antigen-antibody interactions as they apply to species identification and drug identification.

  • Perform a double diffusion precipitin test to identify the species origin of a biological sample.

  • Distinguish between monoclonal and polyclonal antibodies.

  • Describe the laboratory tests necessary to characterize seminal stains.

  • Explain how to properly preserve suspect blood and semen stains for laboratory examination.

  • Describe the correct collection techniques for physical evidence in a rape investigation.

2. Introduction

  • Forensic serologists conduct chemical and biological tests on biological samples from crime scenes to identify the source of the sample.

  • This information can either link a suspect to a crime scene or indicate that they were not present during the event.

  • An example includes determining whether blood found on a suspect’s sock belongs to the victim or if blood found under the victim's fingernails belongs to the suspect.

  • This process involves serological analysis focusing on blood, but knowledge of semen analysis is also crucial for investigations.

  • Blood consists of various components:

    • The liquid portion called plasma includes:

    • Proteins

    • Hormones

    • Clotting agents

    • Electrolytes

    • Different forms of cells

    • The three types of blood cells are:

    • Leucocytes (white blood cells)

    • Erythrocytes (red blood cells)

    • Platelets

    • Serum, the yellowish substance left after blood has clotted, contains plasma proteins and enzymes but lacks cells and clotting factors.

3. The ABO System

  • From a forensic serologist's perspective, RBCs and serum are vital for blood analysis.

  • Red blood cells (RBCs) contain antigens that define blood type.

  • The four blood types determined by antigens are:

    • Type A: Has A antigens on RBCs.

    • Type B: Has B antigens on RBCs.

    • Type AB: Has both A and B antigens on RBCs.

    • Type O: Has no A or B antigens on RBCs.

  • The Rh factor (D antigen) further classifies blood into Rh positive or Rh negative based on the presence of this additional antigen:

    • e.g., A-positive blood contains both A antigens and the Rh factor.

    • O-negative blood lacks A, B, and Rh antigens.

  • Blood serum has specialized proteins known as antibodies, which are highly selective and react only with specific antigens, forming antigen-antibody complexes.

  • Each blood sample is examined independently to determine its type, and sufficient and viable samples are necessary for testing.

  • Various factors can degrade blood samples, including:

    • Putrefaction from bacterial activity

    • Temperature and moisture affecting degradation

  • Dry samples can minimize bacterial growth but are less optimal for DNA profiling.

4. Presumptive Tests

  • Presumptive tests are divided into two categories:

    • Color reaction tests

    • Fluorescent reaction tests

  • Examples of color change tests include:

    • Kastle-Meyer Colour Test: Uses phenolphthalein which reacts with hydrogen peroxide and hemoglobin to produce a dark pink color. Fast results but can yield false positives with certain vegetables.

    • Tetramethylbenzidine (TMB): Requires a moistened swab tested against a dipstick that changes color in the presence of blood.

    • Leucomalachite Green (LMG): Produces a green color during blood chemical reactions, established since the early 20th century.

  • Fluorescent tests expose invisible blood under UV light using chemicals sprayed on suspected areas:

    • Luminol: Extremely sensitive, can detect blood in very low quantities and even on washed surfaces. Potential interference with subsequent testing should be noted.

    • Fluorescein: Preferred when bleach cleaning agents are involved, sticks better to vertical surfaces.

  • Confirmatory tests include:

    • Teichmann Test: Identifies hemoglobin crystals under a microscope.

    • Takayama Test: Similar purpose, filtering through reagents to form observable crystals.

    • Rapid Stain Identification of Human Blood (RSID): Efficient, quick tests for blood as well as other biological fluids.

5. Determining Human Blood

  • Tests determining the source of blood use antigen-antibody reactions specific to human traits.

  • Antiserum is produced from exposing an animal to human antigens, which allows for the designation of human blood through visible precipitation reactions.

  • Absorption-elution is a four-step method to extract and analyze antigens from dried blood stains. The steps are:

    1. Treat blood-stained material with an antiserum.

    2. Wash to remove excess antibodies.

    3. Use heat to disrupt the antigen-antibody bond.

    4. Test eluted antibodies against known blood antigens for reactions.

  • Population distribution data for blood types can assist investigators in narrowing suspects based on blood evidence.

  • Beyond ABO types, additional proteins and antigens (e.g., Duffy, Kell, Kidd, enzymes) can further specify blood samples.

6. Principle of Heredity and Blood Groups

  • Four ABO phenotypes present: A, B, AB, O; with varying genotypes influencing traits.

  • The inheritance involves alleles from both parents, with A and B being codominant and O being recessive.

  • Examples of genotype categorization:

    • AA or AO genotypes give type A blood.

    • OO genotype leads to type O blood, which cannot express A or B traits.

  • Paternity testing can exclude certain individuals based on blood type compatibility, emphasizing the need for comprehensive genetic testing (e.g., HLA markers).

7. Testing Other Bodily Fluids

  • Beyond blood, other bodily fluids like semen and saliva play roles in criminal investigations.

  • Common methods for identifying these include UV or laser light examination to reveal the fluids.

  • Testing semen involves both presumptive and confirmatory tests:

    • Acid Phosphatase (AP): The presence of high levels indicates potential semen, but false positives are possible with various substances.

    • Microscopic Examination: A definitive confirmatory method identifying sperm presence in a sample.

    • Prostate-Specific Antigen (PSA): Identifies evidence of semen even when sperm are not present, and remains unaffected by vasectomy conditions.

8. Activity

  • Review questions cover content such as:

    1. Definition and significance of the A-B-O blood system.

    2. The role of antigens and antibodies in blood individualization.

    3. Differences between monoclonal and polyclonal antibodies—monoclonal antibodies are often favored due to specificity.

    4. Importance of environmental factors affecting blood samples.

    5. The purpose of precipitin tests and their strengths.

    6. The implications of blood group inheritance in legal contexts, like paternity testing.

  • Emphasize the necessity for careful collection and preservation of biological evidence at crime scenes to maintain integrity for analysis.