Immunological Assays in Forensic Science

Module Learning Objectives and Session Overview

• Module Learning Objective 1 (MLO1): Discuss the structure and specificity of antibodies.

• Session Objectives:

  • Explain the fundamental principles underlying immunoassays.

  • Describe the specific applications of immunological assays in forensic sciences, specifically focusing on:

    • Binding assays.

    • Precipitation assays.

    • Agglutination assays.

• Fundamental Principle: The immune system produces a specific antibody in response to exposure to a particular antigen. Immunoassays leverage this highly specific antigen-antibody interaction and reaction to detect analytes.

Principles of Immunity

• Innate (Non-Specific) Immunity:

  • Recognition: Utilizes Pathogen-Associated Molecular Patterns (PAMPs) and Pattern Recognition Receptors (PRRs).

  • Physical Barriers: Provides the first line of defense.

  • Inflammation: Involves histamine release which leads to the dilation of blood vessels, facilitating the entry of phagocytes.

  • Phagocytosis: The process of engulfing and destroying pathogens.

  • Cellular Components:

    • Neutrophils

    • Eosinophils

    • Monocytes

    • Macrophages

    • Dendritic cells

    • Natural killer cells

    • Mast cells

  • Action: Direct killing of cells.

• Adaptive (Specific) Immunity:

  • Recognition: Based on specific interaction between Antibodies and Antigens.

  • Fluid Defense: Mediated by T-cells and B-cells.

  • Activation and Replication:

    • B-cell lineage: Leads to the replication of Memory B-cells and Plasma cells. Plasma cells secrete antibodies to recruit további immune responses.

    • T-cell lineage: Leads to the replication of Memory T-cells and Cytotoxic T-cells. Cytotoxic T-cells attack pathogens directly via apoptosis (programmed cell death).

Fundamentals of Immunoassays

• Definition: Bioanalytical methods where the detection of an analyte depends on the reaction of an antigen with a specific antibody.

• Indication of Presence: The occurrence of binding suggests the presence of a specific analyte within a sample.

• Forensic Application Example (Toxicology): In specific toxicological screenings, antibodies are used to bind to opiates to confirm their presence.

• In Vivo vs. In Vitro Outcomes: Interaction between antigens and antibodies (both in the body and in laboratory settings) results in three primary outcomes:

  1. Neutralization

  2. Precipitation

  3. Agglutination

• In Vitro Mirroring: Diagnostic immunoassays mirror these natural interactions to provide expected results in the presence of a specific antigen through binding, precipitation, or agglutination assays.

1. Binding Assays

• Overview:

  • Binding occurs directly between the antibody and the antigen.

  • A detection system is typically employed to visualize the binding event.

  • Quantification: These assays are quantitative; the amount of signal or fluorescence detected is directly proportional to the amount of the substance present in the sample.

  • Common Example: Over-the-counter pregnancy tests.

• Typical Binding Assay Types:

  • Western Blotting:

    • Assesses specific proteins present in cellular or tissue samples.

    • Process Steps:

      1. Electrophoresis: Separation of proteins by size.

      2. Transfer: Moving proteins from the gel to a membrane (utilizing a sandwich of sponge, filter paper, gel, and membrane).

      3. Blocking: Using a blocker to prevent non-specific binding on the membrane.

      4. Primary Antibody Incubation: The primary antibody binds to the target antigen.

      5. Washing: To remove unbound primary antibodies.

      6. Secondary Antibody Incubation: Secondary antibody binds to the primary antibody.

      7. Detection: Visualization of the protein bands (measured in weight units such as $kDa$).

  • Indirect Immunofluorescence:

    • Uses monoclonal primary antibodies to bind to the antigen.

    • A monoclonal secondary antibody, bound to a fluorescent label (fluorophore), specifically binds to the primary antibody.

    • The resulting binding is visualized as fluorescence under a microscope.

  • Immunohistochemistry (IHC):

    • Used for the detection, localization, and expression profiling of proteins in tissues or cells.

    • Follows a similar process to indirect immunofluorescence but does not use fluorescence.

    • Uses a polyclonal secondary antibody linked to an enzyme.

    • The enzyme catalyzes a reaction that causes the release of a dye.

    • Forensic Applications: Used in post-mortem diagnosis to determine if a condition was a natural disease and has potential for use as vitality markers for wound aging.

  • Enzyme-linked Immunosorbent Assay (ELISA):

    • An antibody is attached to an enzyme.

    • The enzyme reacts with a substrate to produce a colored product.

    • A color change indicates that the antigen or antibody is present.

    • Quantification: Color intensity is directly proportional to the quantity of antigen present.

2. Precipitation Assays

• Principle: Involves the interaction between soluble antigens and antibodies. When they bind, they form an insoluble precipitate.

• Mechanism: Multiple antigens and antibodies link together to form a complex lattice structure. The more interactions that occur, the more visible the precipitate becomes.

• Forensic Application: The Precipitin Test (1897):

  • Used to identify the species of origin for biological samples.

  • Method: Antibodies from human serum samples are mixed with an unknown blood sample. If human antigens are present, an antigen-antibody complex (precipitate) forms.

  • Historical Case Study: Robert Handley (1926). Handley was convicted for the rape and murder of Euphemia Shanne. His conviction, resulting in a sentence of $15\,\text{years}$, was based on circumstantial evidence and positive precipitin tests confirming the presence of human blood on his raincoat sleeve.

3. Agglutination Assays

• Principle: Interaction between an antibody and an antigen located on the surface of a cell (or particulate), resulting in the formation of visible clumps.

• Distinction from Precipitation: Agglutination involves the formation of "clumps" (cell-bound) rather than a "lattice" (soluble).

• Comparison of Agglutination and Precipitation:

  • Agglutination: Involves bacteria or cells, typically uses $IgM$ antibodies, and results in a suspension of clumps.

  • Precipitation: Involves soluble epitopes/antigens in a solution and results in a precipitate/supernate separation.

• Advantages:

  • Easy to perform.

  • Inexpensive equipment requirements.

  • Requires only a small amount of antibody.

• Forensic Application: Haemagglutination:

  • Used for blood typing.

  • Tests Red Blood Cells (RBC) for the presence of specific surface antigens.

  • It remains the most commonly used immunoassay in the world.