Notes on Antibody-Based Diagnostics and Assays

Clinical Importance of Serology

  • Serology significance: Crucial for detecting antibodies in patients to diagnose infections, autoimmune diseases, and malignancies.

Key Definitions

  • Antibody Titer: Relative concentration of antibodies in a patient's serum, measured as the reciprocal of the last dilution causing a detectable reaction.

Diagnostic Assays Overview

  • Main types of immunodiagnostic assays include:
    • Precipitation reactions: Antigen-antibody interactions lead to insoluble complexes.
    • Agglutination reactions: Antibodies reacting with particulate antigens leads to clumping.
    • Labeled immunoassays: Antibodies or antigens labeled for visualization or quantification.
  • Each assay measures either antigen or antibody presence.

Precipitation Reactions

  • Mechanism:
    • Involves soluble antigens and antibodies forming insoluble precipitates.
  • Key Conditions: Optimal concentrations are crucial for precipitation, referred to as the equivalence zone.
    • Test Types:
    • Radial Immunodiffusion: Measures antigens by diameter of precipitin rings.
    • Double Immunodiffusion: Antibody and antigen diffusion into agar leads to a visible precipitin line.

Agglutination Reactions

  • Mechanism:
    • Soluble antibodies react with insoluble antigens creating visible clumps.
  • Examples:
    • Hemagglutination: Used for blood typing (e.g., ABO types), visible as clumps or buttons of RBCs.
  • Optimal conditions: Serial dilutions help achieve optimal concentration.
  • Applications: Diagnosis of infections like Salmonella.

Labeling Techniques

  • Purpose: Attach labels to antibodies for detection (e.g., radioactivity, enzymes).
  • Types of labels:
    • Radioisotopes (e.g., iodine-125), enzymes (e.g., horseradish peroxidase), fluorescent dyes.
  • Common Assays Utilizing Labels:
    • ELISA - quantifies antibodies using enzyme-linked detection methods.
    • Western Blot - protein separation and detection to identify specific antibodies (e.g. HIV).

Immunofluorescence Assays

  • Types:
    • Direct: Primary antibodies labeled with fluorescent dyes.
    • Indirect: Secondary antibodies bind to primary, aiding visualization.
  • Common Uses: Detection of rabies viruses, autoimmunities like RA.

Flow Cytometry/FACS

  • Technique: Uses fluorescent-tagged antibodies to analyze cell populations.
  • Applications: Enumeration of specific WBC subsets, significant for leukemia diagnosis.

SARS-CoV-2 Testing

  • Different strategies for detection:
    • Nucleic acid testing: PCR methods for detecting viral genome.
    • ELISA Tests: For antibodies and antigen detection, with at-home kits becoming widespread.

Summary of Key Concepts

  • Immunoassays types include precipitation, agglutination, and labeled methods, each critical for diagnosing various diseases.
  • Successful diagnosis often hinges on the accurate determination of titers and use of specific assays.