Lab Analysis of the Immune Response - MMG201 Chapter 20

Overview of Lab Analysis of the Immune Response

• Chapter 20: Focuses on Laboratory Analysis of the Immune Response.

• Related Material:
      - Chapter 17: Innate nonspecific host defenses.
      - Chapter 18: Adaptive specific host defenses.

Principles of Antigen-Antibody Detection

• Review of Antigens and Antibodies:
      - Antigen-binding sites: Areas on the antibody that bind to specific epitopes.
      - Epitopes: Also known as antigenic determinants; specific parts of an antigen molecule to which an antibody attaches.
      - Specificity: Multiple antibodies (e.g., Antibody A, B, and C) can target different epitopes on the same antigen.

• Detecting Antigen-Antibody (Ag-Ab) Complexes:
      - Diagnostics test for two primary possibilities in a patient serum/sample:
          1. Antigens for a specific pathogen.
          2. Antibodies to a specific antigen linked to a pathogen.

• Clinical Implications:
      - The presence of antibodies suggests an adaptive specific host defense has been activated.
      - The presence of antigens suggests an active infection or presence of the pathogen.
      - In a primary exposure event, the timing and type (IgM vs. IgG) indicate the stage of the immune response.

• In Vitro Assays:
      - When both antibodies and corresponding antigens are present in a solution, they can form large complexes called lattices.
      - These lattices settle out of solution in a process known as a precipitation reaction.

• Antiserum Types:
      - Monoclonal antibody serum: Binds to a single specific epitope.
      - Polyclonal antibody serum: Binds to many different epitopes on an antigen.

Precipitation Reactions

• Overview: An antibody binds to a soluble antigen to form a visible precipitin.

• Purpose: Qualitative or quantitative detection of antibodies or antigens.

• Qualitative Assays (Presence/Absence):
      - Precipitin Ring Test:
          - Performed in test tubes or capillary tubes.
          - Uses a glycerol layer to prevent mixing of the standard antigen solution and the antiserum.
          - Precipitation occurs in the middle where they meet.
          - Issues: Requires large serum volumes; sensitive to disruption of the ring.
      - Ouchterlony Test (Double Immunodiffusion):
          - Uses an agar/gel medium with holes punched into it.
          - The center hole typically contains the antigen, while surrounding holes contain antiserum.
          - Both diffuse through the agar; an arc (precipitin arc) forms at the zone of equivalence.
          - Useful for checking cross-reactivity against closely related antigens.
      - Flocculation:
          - Antibody binds to insoluble molecules (e.g., lipids) in suspension, forming visible aggregates.
          - VDRL Test (Venereal Disease Research Lab): An indirect method used for Syphilis (Treponema pallidum). Because T. pallidum is difficult to grow and too small for standard microscopy, this test detects antibodies to treponemal antigens.

• Quantitative Assays (Measuring Concentration):
      - Radial Immunodiffusion (RID):
          - Antigen and antibody interact in an agar/gel medium to form a ring of precipitation.
          - The size of the zone of precipitation is measured, similar to a zone of inhibition in disk diffusion.
          - Used to determine the concentration of serum proteins and complement proteins.

Specialized Immunoassays

• Neutralization Assay:
      - Antibodies bind to a virus, blocking its entry into target cells and preventing plaque formation.
      - Interpretation: A high number of plaques indicates a LOW concentration of neutralizing antibodies.
      - Example: Neutralization of SARS-CoV-2 spike protein in COVID-19 patients and convalescent plasma donors.

• Complement-mediated Immunoassay (Complement Fixation Test):
      - Used to detect antibodies against pathogens difficult to culture (fungi, viruses, or Chlamydia).
      - Mechanism: Antibody binds to antigen, inducing complement activation.
      - Negative Result: If no antibodies are present, complement remains free to lyse added red blood cells (RBCs).
      - Positive Result: If antibodies are present, complement is "fixed"/used up, leaving no complement to lyse RBCs (solution stays cloudy/pink).

• Immunoblot Assay (Western Blot):
      - Employs enzyme-antibody conjugates to identify specific proteins on an absorbent membrane following an immunoelectrophoresis step.
      - Example: Lyme Disease Diagnosis:
          - Tests for IgM and IgG antibodies.
          - Controls: P (Positive), C (Calibrator), N (Negative).
          - Interpreting results (e.g., Hiker 1): Detectable IgG with undetectable IgM suggests a past exposure (assuming a three-week timeline where IgM would have already declined).

Agglutination Assays

• Definition: Clumping (agglutination) of cells or particles caused by antibodies binding to them.

• Direct Agglutination Assay:
      - Bacterial cells clump on their own when exposed to antibodies (e.g., targeting the M protein virulence factor in Streptococci).
      - Direct Hemagglutination (Direct Coombs’/Antiglobulin Test): Detects antibodies already adhered to the surface of red blood cells (e.g., immune-mediated hemolytic anemia).

• Indirect Agglutination Assay (Latex Fixation):
      - Uses inert latex beads coated with either antigens or antibodies as indicators.
      - Latex beads coated with Antigen: Detects antibodies in patient serum.
      - Latex beads coated with Antibodies: Detects specific antigens in patient serum.

• Viral Hemagglutination (Influenza):
      - Observed when host samples containing hemagglutinin (H) spike proteins are mixed with RBCs, causing clumping.

• ABO Blood-Typing:
      - Uses a bedside card or solution coated with manufactured Anti-A and Anti-B antibodies.
      - Positive Result: Clumping (agglutination) between the targeted RBC antigen and the added antibody.
      - A-type blood agglutinates with Anti-A solution; B-type blood agglutinates with Anti-B solution.

Enzyme-Linked Immunosorbent Assays (ELISA) and Immunostaining

• Enzyme Immunoassay (EIA/ELISA):
      - Uses enzyme-antibody conjugates to quantify target molecules through color change.
      - Direct ELISA: Uses a single antibody-enzyme conjugate to detect antigen.
      - Indirect ELISA: Uses a primary antibody to bind antigen and a secondary enzyme-linked antibody to bind the primary antibody.
      - Sandwich ELISA: Uses a capture antibody to fix the antigen, then a detection antibody.
      - Competitive ELISA: Uses an inhibitor to compete with the antigen for binding sites.
      - Application: HIV ELISA test uses a dilution series (from $1:100$ to $1:12800$) to determine reactivity.

• Immunostaining:
      - Immunohistochemistry (IHC): Used for examining whole tissues (e.g., cardiac epidermoid carcinoma, bone marrow).
      - Immunocytochemistry (ICC): Used for examining specific intracellular structures; the antibody passes through the cell membrane to bind targets like organelles or the cytoskeleton.

• Immunochromatographic / Immunofiltration Assays:
      - Lateral flow tests: Quick, user-friendly diagnostic tools using colored antigen-antibody complexes.
      - Examples: SARS-CoV-2 rapid tests and Pregnancy tests (detecting hormones produced by the placenta in urine).

Fluorescent Antibody Techniques

• Direct Fluorescent Antibody (DFA) Test:
      - Uses a fluorescently labeled monoclonal antibody (mAb) to illuminate a target antigen directly.
      - Applications: Rapid diagnosis of Streptococcus pyogenes (Strep throat) from swabs, or pneumonia (Mycoplasma pneumoniae, Legionella pneumophila) from sputum.

• Indirect Fluorescent Antibody (IFA):
      - Used to diagnose Syphilis by detecting antibodies in the sample using T. pallidum cells isolated from lab animals as the target.

Questions & Classroom Discussion

• Class Question: Which of the following samples do you predict contains polyclonal antibodies?
      - Context: The discussion refers to the science behind Ag-Ab complex detection and agglutination.

• ALE Question 1: Involved interpreting results from a Flocculation Test (RPR-Carbon).

• ALE Question 2: Which assay(s) allow for detection of the Ag-Ab complex but cannot provide measurement (quantification) of concentrations?
      - Answer context: These are the Qualitative assays: Precipitin ring test (mostly qualitative), Ouchterlony test, and Flocculation.

• ALE Question 3 (Hiker Example):
      - Scenario: A hiker on a 3-week trip is tested for Lyme disease.
      - Finding: IgM not detected; IgG detected.
      - Interpretation: IgM is the first antibody produced in a primary response; its absence alongside presence of IgG suggests the exposure occurred long enough ago for the IgM to decline to undetectable levels.

• Class Question: What do you need to ADD to a blood sample to CAUSE an agglutination reaction so you can visualize the results?
      - Answer: A manufactured antibody serum (anti-antigen solution).

• Class Discussion Question: What assay(s) could you perform to determine if a person was exposed to a bacterial or eukaryotic pathogen three (3) months ago?
      - Options Mentioned: ELISA (to detect antibodies), Western blot, Neutralization test.

• Note on PCR (Polymerase Chain Reaction): While not the primary focus of this chapter (Chapter 12 material), Chapter 20 notes its use in modern lab analysis for identifying gene targets in samples.