Antibody-Based Assays and Techniques Summary

Antibody Specificity and Cross-Reactivity

• Antibodies exhibit specificity for antigens but can cross-react with similar antigens.

Polyclonal vs. Monoclonal Antibodies

• Polyclonal Antibody Response:
  • Injecting an antigen into an animal induces a polyclonal response.
  • This generates a variety of antibodies reacting with different epitopes on the same antigen.
  • Useful for some lab assays and screening.
  • Limitations: Risk of false positives and negatives.
• Monoclonal Antibodies:
  • Higher specificity, binding to a single epitope with high affinity.
  • Produced from hybridomas derived from mice.
  • Applications: Cancer treatment.
  • Limitations: High cost restricts wider use for infectious diseases.
  • Research is focused on cost-effective alternatives like plantibodies.

Precipitation Reactions

• Precipitin Formation:
  • Occurs when antibody and antigen are in optimal proportions, forming a lattice.
  • This lattice precipitates out of the solution.
• Precipitin Ring Test:
  • Visualizes lattice formation in solution.
• Ouchterlony Assay:
  • Demonstrates precipitin formation in a gel.

Western Blot Assays

• Principle:
  • Used to identify specific antigens in serum.
  • Proteins are transferred to a nitrocellulose membrane.
  • Detected using labeled antibodies.

Complement Fixation Test

• Principle:
  • Utilizes complement to detect antibodies against pathogens.

Agglutination Reactions

• Agglutination:
  • Antibodies agglutinate cells or large particles into a visible matrix.
  • Tests are often performed on cards or in microtiter plates.
  • Small reagent volumes are used for multiple reactions.
• Diagnostic Value:
  • Detecting antibodies against a pathogen is a valuable diagnostic tool.
  • Issue: Delay due to seroconversion (time before antibodies are detectable).
• Indirect Agglutination Assays:
  • Utilize agglutination of latex beads to detect specific antigens or antibodies in patient serum.
• Direct Coombs' Test:
  • Confirms the presence of antibacterial and antiviral antibodies.
  • Coombs' reagent cross-links antibodies on red blood cells, facilitating hemagglutination.

Hemagglutination Assays

• Applications:
  • Screening and cross-matching donor and recipient blood.
  • Ensuring compatibility during transfusions.
  • Preventing adverse reactions.

Enzyme Immunoassays (EIA)

• Principle:
  • Used to visualize and quantify antigens.
  • Employs an enzyme-conjugated antibody.
  • Antibody binds to the antigen.
  • Enzyme converts a substrate into an observable product (chromogen or fluorogen).

Immunostaining

• Technique within EIA:
  • Immunohistochemistry: Visualizing cells in tissue.
  • Immunocytochemistry: Examining intracellular structures.

ELISA Types

• Direct ELISA:
  • Quantifies an antigen in solution.
  • Primary antibody captures the antigen.
  • Secondary antibody delivers an enzyme.
  • Product formation is proportional to the amount of captured antigen.
• Indirect ELISA:
  • Detects antibodies in patient serum.
  • Antigen is bound to a microtiter plate.
  • Primary antibody binds to the antigen . Followed by detection with an enzyme-conjugated secondary antibody.

Immunofiltration and Immunochromatographic Assays

• Applications:
  • Used in lateral flow tests.
  • Diagnose pregnancy and various diseases.
  • Detect color-labeled antigen-antibody complexes.
  • Samples: Urine or other fluid samples.

Immunofluorescence Assays

• Principle:
  • Utilize antibody-fluorogen conjugates.
  • Allow for easy and rapid detection of antigens.
• Direct Immunofluorescence:
  • Can detect the presence of bacteria in clinical samples (e.g., sputum).

Flow Cytometry

• Principle:
  • Employs fluorescent monoclonal antibodies (mAbs) against cell-membrane proteins.
  • Quantifies specific subsets of cells within complex mixtures.

Fluorescence-Activated Cell Sorters (FACS)

• Extension of Flow Cytometry
  • Uses fluorescence intensity to physically separate cells.
  • Separates into high and low fluorescence populations.