Unit 3: Antigen - antibody interactions - Subjective Questions

Antibody Affinity:

• Affinity refers to the strength of interaction between a single antigen-binding site (paratope) and a single epitope.
• It is a measure of the sum of attractive and repulsive forces (hydrogen bonds, electrostatic interactions, van der Waals forces, and hydrophobic forces) between one Fab site and one epitope.

The Affinity Equation:

The interaction can be represented as a reversible reaction:

The association (affinity) constant is:

• A high indicates strong binding (high affinity).
• Typical values range from to .

Antibody Avidity:

• Avidity is the overall strength of binding between a multivalent antibody and a multivalent antigen.
• It accounts for the total stability of the antigen-antibody complex, involving multiple binding sites.

Key Differences:

• IgM, though having low affinity per site, has high avidity due to its 10 binding sites.

Cross-Reactivity:

Cross-reactivity occurs when an antibody raised against one specific antigen binds to a different antigen that shares structurally similar or identical epitopes.

Mechanism:

• Antibodies recognize epitopes, not whole antigens.
• If two different antigens share identical or similar epitopes, an antibody can bind to both.
• Cross-reactivity may also occur due to structural similarity where the antibody binding site accommodates both.

Examples:

• ABO Blood Group System: Antibodies against intestinal microbiota carbohydrates cross-react with A and B blood group antigens.
• Vaccinia (cowpox) virus: Provides immunity against smallpox due to shared epitopes — the basis of Jenner's vaccine.
• Streptococcus pyogenes: Antibodies cross-react with heart tissue causing rheumatic fever.

Significance:

• Positive uses:
  • Basis for certain vaccinations (e.g., smallpox).
  • Useful in developing diagnostic assays.
• Negative implications:
  • Can lead to autoimmune diseases (molecular mimicry).
  • Causes false-positive results in serological tests.

Conclusion: Cross-reactivity is a double-edged sword—beneficial for immunity but potentially harmful in autoimmunity and diagnostic accuracy.

Precipitation Reaction:

A precipitation reaction occurs when a soluble antigen reacts with a soluble antibody in the presence of electrolytes to form an insoluble visible precipitate (lattice).

Lattice Hypothesis (Marrack):

• Formation of large antigen-antibody lattices requires multivalent antigens and bivalent (or multivalent) antibodies.
• Cross-linking creates a network that precipitates out of solution.

The Precipitin Curve:

When increasing amounts of antigen are added to a fixed amount of antibody, the amount of precipitate follows a bell-shaped curve with three zones:

1. Zone of Antibody Excess (Prozone):

• Excess antibody, little antigen.
• Small complexes form; little or no precipitate.
• Free antibody remains in the supernatant.

2. Zone of Equivalence:

• Optimal ratio of antigen to antibody.
• Maximum lattice formation and maximum precipitation.
• Neither free antigen nor free antibody in supernatant.

3. Zone of Antigen Excess (Postzone):

• Excess antigen saturates antibody binding sites.
• Small soluble complexes form; decreased precipitate.
• Free antigen remains in supernatant.

Clinical Importance (Prozone Phenomenon):

• False-negative results occur in tests when antibody is in excess (e.g., in secondary syphilis serology). Sample dilution resolves this.

Immunodiffusion refers to precipitation reactions carried out in a semi-solid gel medium (agar/agarose), allowing antigen and antibody to diffuse and form visible precipitin lines/rings.

1. Single Radial Immunodiffusion (Mancini Technique):

• Antibody is uniformly incorporated into the agar gel.
• Antigen is placed in wells and diffuses radially outward.
• A ring of precipitation forms where equivalence is reached.
• The diameter (or area) of the ring is proportional to antigen concentration:

• Use: Quantitative estimation of antigens (e.g., serum immunoglobulins).

2. Double Immunodiffusion (Ouchterlony Technique):

• Both antigen and antibody diffuse toward each other from separate wells.
• Precipitin lines form at the zone of equivalence.
• Use: Qualitative — determines antigenic relationships.

Patterns in Ouchterlony:

• Identity (arc/fusion): Antigens are identical.
• Non-identity (crossed lines): Antigens are unrelated.
• Partial identity (spur): Antigens share some but not all epitopes.

Comparison Table:

Agglutination Reaction:

Agglutination is the visible clumping of particulate antigens (cells or particles) by antibodies. The antibody involved is called an agglutinin.

Key Feature: Unlike precipitation (soluble antigen), agglutination involves particulate/insoluble antigens.

1. Direct (Active) Agglutination:

• Antigens are naturally present on the surface of particles (e.g., RBCs, bacteria).
• Antibody directly agglutinates these particles.
• Examples:
  • Blood typing (ABO grouping): Antibodies clump RBCs bearing A or B antigens.
  • Widal test: Detecting antibodies against Salmonella antigens.

2. Indirect (Passive) Agglutination:

• Soluble antigens (or antibodies) are artificially coated onto inert carrier particles (latex beads, RBCs).
• Agglutination then occurs.
• Examples:
  • Latex agglutination test for detecting Rheumatoid Factor (RF), CRP.
  • Hemagglutination using antigen-coated RBCs.

Reverse Passive Agglutination:

• Antibody (instead of antigen) is coated on the carrier particle to detect soluble antigen.

Prozone Effect:

• Excess antibody can inhibit agglutination, causing false negatives — resolved by serial dilution.

Applications: Blood grouping, pregnancy tests, bacterial identification, and detection of autoantibodies.

Radioimmunoassay (RIA):

RIA is a highly sensitive competitive binding assay developed by Rosalyn Yalow and Solomon Berson (Nobel Prize 1977) used to measure minute concentrations of antigens/hormones.

Principle:

• Based on competition between *radiolabeled antigen (Ag) and unlabeled (test) antigen (Ag)** for a limited amount of specific antibody.

• More unlabeled antigen present → less labeled antigen binds to antibody.
• Bound radioactivity is inversely proportional to the concentration of unlabeled (test) antigen.

Procedure:

1. Mix fixed amounts of antibody and radiolabeled antigen with the test sample.
2. Allow competitive binding to equilibrium.
3. Separate bound from free antigen (precipitation/second antibody).
4. Measure radioactivity of bound fraction using a gamma counter.
5. Determine unknown concentration from a standard curve.

Radioisotopes used: , , , .

Applications:

• Measurement of hormones (insulin, thyroid hormones).
• Detection of drugs, vitamins, and viral antigens (Hepatitis B).

Advantages:

• Extremely high sensitivity (picogram levels).
• High specificity.

Limitations:

• Radiation hazard and disposal issues.
• Short shelf-life of reagents.
• Requires specialized equipment and licensing.
• Largely replaced by ELISA.

ELISA (Enzyme-Linked Immunosorbent Assay):

ELISA is a sensitive immunological assay using an enzyme-labeled antibody or antigen to detect and quantify antigens or antibodies. The enzyme acts on a substrate to produce a measurable colored product.

Common Enzymes & Substrates:

• Horseradish Peroxidase (HRP) — substrate: TMB/OPD.
• Alkaline Phosphatase (ALP) — substrate: pNPP.

Types of ELISA:

1. Direct ELISA:

• Antigen is coated on the plate.
• An enzyme-linked primary antibody binds directly.
• Simple but less sensitive.

2. Indirect ELISA:

• Antigen coated on plate.
• Unlabeled primary antibody binds antigen.
• Enzyme-linked secondary antibody binds the primary antibody.
• Used for antibody detection (e.g., HIV screening).

3. Sandwich ELISA:

• Capture antibody coated on plate binds antigen.
• Antigen is 'sandwiched' by a second enzyme-linked detection antibody.
• Used for antigen detection; highly specific (requires antigen with ≥2 epitopes).

4. Competitive ELISA:

• Sample antigen competes with a reference antigen for antibody binding.
• Signal is inversely proportional to sample antigen concentration.

Measurement:

• Color intensity measured by a spectrophotometer (ELISA reader) — optical density (OD) proportional (or inversely) to analyte concentration.

Applications:

• Diagnosis of HIV, Hepatitis, Dengue.
• Hormone and cytokine quantification.
• Detection of autoantibodies.

Immunofluorescence (IF):

A technique that uses antibodies conjugated to fluorescent dyes (fluorochromes) such as FITC (green) or Rhodamine (red) to visualize the location of antigens using a fluorescence microscope.

1. Direct Immunofluorescence (DIF):

• A fluorochrome-labeled primary antibody binds directly to the target antigen.
• Single-step procedure.
• Diagram concept: Antigen ← Fluorescent-labeled Antibody.
• Advantages: Rapid, less background/nonspecific staining.
• Disadvantages: Less sensitive (no signal amplification); needs a specific labeled antibody for each target.
• Applications: Detecting immune complex deposits in skin/kidney biopsies (e.g., in SLE, pemphigus).

2. Indirect Immunofluorescence (IIF):

• An unlabeled primary antibody binds the antigen.
• A fluorochrome-labeled secondary antibody binds the primary antibody.
• Two-step procedure.
• Diagram concept: Antigen ← Primary Ab ← Fluorescent-labeled Secondary Ab.
• Advantages: More sensitive (signal amplification — multiple secondary antibodies bind one primary); one labeled secondary antibody works for many primaries.
• Disadvantages: More steps; higher background staining.
• Applications: Detection of autoantibodies (ANA test for SLE), antibody titer determination.

Comparison Table:

Western Blotting (Immunoblotting):

A technique to detect specific proteins in a sample by combining gel electrophoresis with antibody-based detection.

Principle:

Proteins are separated by size, transferred to a membrane, and probed with specific antibodies to identify the target protein.

Procedure (Steps):

1. Protein Separation (SDS-PAGE):

• Proteins are denatured and separated by molecular weight using polyacrylamide gel electrophoresis with SDS.

2. Transfer (Blotting):

• Separated proteins are electrophoretically transferred from the gel onto a nitrocellulose or PVDF membrane.

3. Blocking:

• Membrane is blocked with non-fat milk/BSA to prevent nonspecific antibody binding.

4. Primary Antibody Incubation:

• Specific primary antibody binds the target protein.

5. Secondary Antibody Incubation:

• Enzyme-labeled (HRP/ALP) secondary antibody binds the primary antibody.

6. Detection:

• Addition of substrate produces a colored or chemiluminescent band at the position of the target protein.

Significance/Applications:

• Confirmatory test for HIV (following a positive ELISA screen).
• Diagnosis of Lyme disease, Hepatitis C.
• Detection of specific proteins in research (protein expression studies).
• Determination of protein molecular weight and abundance.

Advantage: High specificity; identifies a specific protein among many.

Immunohistochemistry (IHC):

IHC is a technique used to detect and localize specific antigens (proteins) in tissue sections using labeled antibodies, while preserving the tissue architecture.

Principle:

• Based on the specific binding of antibodies to target antigens in situ within a tissue section.
• The antibody is linked to a detectable label (enzyme or fluorophore) that produces a visible signal at the antigen location.

Procedure (Steps):

1. Tissue Fixation & Sectioning: Tissue fixed (formalin) and embedded (paraffin); thin sections cut and mounted on slides.
2. Antigen Retrieval: Heat/enzyme treatment to unmask epitopes hidden by fixation.
3. Blocking: Prevent nonspecific binding.
4. Primary Antibody: Binds the specific tissue antigen.
5. Detection System:
   • Direct: Labeled primary antibody.
   • Indirect: Labeled secondary antibody binds primary (more sensitive).
6. Visualization:
   • Chromogenic (IHC): Enzyme (HRP) + substrate (DAB) → brown precipitate.
   • Fluorescent (IF): Fluorochrome viewed under microscope.
7. Counterstaining: Hematoxylin to visualize tissue morphology.

Clinical Applications:

• Cancer diagnosis and classification (e.g., HER2, ER/PR in breast cancer).
• Identifying tumor origin and metastasis.
• Detecting infectious agents in tissue.
• Distinguishing between cell types in pathology.

Advantage over IF: Chromogenic IHC provides permanent slides viewable under a light microscope.

Precipitation vs. Agglutination:

Both are secondary antigen-antibody reactions resulting in visible complexes, but they differ in the nature of the antigen.

Comparison Table:

Why Agglutination is More Sensitive:

1. Larger antigen-carrying particles: In agglutination, antigens are attached to large particles (cells/beads). Cross-linking of a few antibodies produces a large, easily visible clump.

2. Amplification effect: Because each particle carries many antigen molecules, even a small number of antibodies can bridge particles into visible aggregates.

3. In precipitation, many antigen-antibody complexes must form before an insoluble lattice becomes visible, requiring larger quantities.

Conclusion: Agglutination can detect lower concentrations of antibody than precipitation, making it more sensitive and widely used in rapid diagnostics.

Non-Covalent Forces in Antigen-Antibody Binding:

Antigen-antibody interactions are non-covalent and reversible. The strength results from the summation of several weak, short-range interactions that require close complementarity (goodness of fit) between epitope and paratope.

1. Hydrogen Bonds:

• Formed between hydrophilic groups (–OH, –NH, –COOH).
• Each bond is weak but numerous bonds add significant strength.

2. Electrostatic (Ionic) Forces:

• Attraction between oppositely charged side chains (e.g., –NH₃⁺ and –COO⁻).
• Governed by Coulomb's law; strength inversely proportional to distance squared:

3. Van der Waals Forces:

• Weak attractions from interaction of electron clouds (induced dipoles) between molecules.
• Effective only at very close range.

4. Hydrophobic Interactions:

• Nonpolar groups associate to exclude water, contributing up to 50% of total binding strength.

Importance of Fit:

• These forces operate only over very short distances.
• Therefore, a close 3D complementarity between antigen and antibody is essential — the closer the fit, the stronger the cumulative interaction and the higher the affinity.

Conclusion: The overall binding strength (affinity) is the net result of these attractive forces minus any repulsive forces, dependent on structural complementarity.

Immunoelectrophoresis (IEP):

A technique combining electrophoretic separation of antigens with immunodiffusion for qualitative analysis of complex protein mixtures (e.g., serum).

Procedure:

1. A complex antigen mixture is placed in a well and separated by electrophoresis in agar according to charge/size.
2. A trough is cut parallel to the separation and filled with antiserum.
3. Antibodies and separated antigens diffuse toward each other.
4. Precipitin arcs form at zones of equivalence for each antigen-antibody pair.

Applications:

• Detection of abnormal immunoglobulins (e.g., myeloma paraproteins).
• Analysis of serum proteins.

Rocket Immunoelectrophoresis (Laurell Technique):

A quantitative electrophoretic technique.

Procedure:

1. Antibody is incorporated uniformly into the agarose gel.
2. Antigen samples are placed in wells and driven through the gel by an electric field.
3. As antigen migrates, it forms rocket-shaped (cone) precipitin peaks.
4. The height of the rocket is directly proportional to the antigen concentration:

Applications:

• Rapid quantification of specific antigens/proteins.

Comparison: IEP is qualitative (identification), while Rocket IEP is quantitative (measurement).

Scatchard Analysis of Antibody Affinity:

The Scatchard equation is used to determine the affinity constant () and the valency (n) of an antibody from equilibrium binding data.

Derivation:

Consider the reversible binding:

The association constant:

Let:

• = moles of antigen (ligand) bound per mole of antibody
• = concentration of free ligand
• = number of binding sites (valency)

The binding relationship is:

Rearranging to the linear Scatchard form:

Interpretation of the Scatchard Plot:

Plotting (y-axis) against (x-axis) gives a straight line for homogeneous antibodies:

• Slope = → gives the affinity constant.
• X-intercept (where ) = → gives the valency (number of binding sites; e.g., 2 for IgG).
• Y-intercept = .

Significance:

• A linear plot indicates a homogeneous population with uniform affinity.
• A curved plot indicates a heterogeneous antibody population with varying affinities.

Conclusion: The Scatchard plot is a powerful tool to quantify both the strength (affinity) and valency of antibody-antigen interactions.

Comparison of RIA and ELISA:

Both are highly sensitive quantitative immunoassays; the key difference is the type of label used.

Comparison Table:

Why ELISA Replaced RIA:

1. Safety: ELISA uses non-hazardous enzyme labels, eliminating radiation risks to personnel and the environment.

2. Reagent Stability: Enzyme-labeled reagents have a long shelf-life, whereas radioisotopes decay quickly (short half-life).

3. No special licensing or radioactive waste disposal is required.

4. Cost-effective and easier to automate for high-throughput testing.

5. Comparable sensitivity and specificity to RIA.

Conclusion: While RIA offers excellent sensitivity, its radiation hazards and practical limitations led to ELISA becoming the preferred method in most diagnostic laboratories.

Sandwich ELISA:

A type of ELISA used to detect and quantify antigens, in which the antigen is captured between two antibodies like a 'sandwich'.

Requirement: The target antigen must have at least two distinct (non-overlapping) epitopes to bind two different antibodies.

Steps (Procedure):

1. Coating: A capture antibody specific to the antigen is immobilized on the microplate well.

2. Blocking: Unbound surface is blocked with BSA/milk to prevent nonspecific binding.

3. Sample Addition: Test sample is added; the antigen binds to the capture antibody.

4. Washing: Unbound components are washed away.

5. Detection Antibody: A second enzyme-linked detection antibody binds to a different epitope on the captured antigen — forming the 'sandwich'.

6. Substrate Addition: Substrate is added; the enzyme converts it into a colored product.

7. Measurement: Color intensity (OD) is measured — directly proportional to antigen concentration.

Diagram concept:

Plate → Capture Ab → Antigen → Detection Ab (enzyme) → Substrate → Color

Why Preferred for Antigen Detection:

• High specificity: Uses two antibodies against different epitopes, reducing cross-reactivity.
• High sensitivity: Effective even with impure/complex samples.
• No need to purify antigen beforehand.

Applications: Detection of hormones, cytokines, cardiac markers (Troponin), and viral antigens (Dengue NS1).

Ouchterlony Double Immunodiffusion Patterns:

In the Ouchterlony technique, antigen and antibody diffuse toward each other in agar, forming precipitin lines. The pattern of these lines reveals the antigenic relationship between substances placed in adjacent wells.

Setup: A central well contains antibody; two peripheral wells contain the two antigens being compared.

1. Reaction of Identity:

• The two antigens are immunologically identical (share all epitopes).
• The precipitin lines from both antigens fuse to form a continuous, smooth arc.
• Diagram concept: A smooth curved line joining continuously between the two antigen wells.

2. Reaction of Non-Identity:

• The two antigens are completely unrelated (no shared epitopes).
• The two precipitin lines cross each other independently (form an X).
• Diagram concept: Two lines crossing over.

3. Reaction of Partial Identity:

• The two antigens share some but not all epitopes (cross-reactive antigens).
• The lines fuse but form a spur (extension) pointing toward the antigen with fewer epitopes (the less complex one).
• Diagram concept: A fused arc with a spur projecting over the well of the partially related antigen.

Summary Table:

Application: Used to compare antigens and study cross-reactivity.

Fluorochromes:

Fluorochromes (fluorophores) are dyes that absorb light of one wavelength and emit light of a longer wavelength (fluorescence). When conjugated to antibodies, they allow visualization/detection of antigens.

Common Fluorochromes:

• FITC (Fluorescein isothiocyanate): Absorbs blue, emits green (~520 nm).
• Rhodamine / TRITC: Emits red-orange.
• Phycoerythrin (PE): Emits orange-red; very bright.

Flow Cytometry / FACS (Fluorescence-Activated Cell Sorting):

An automated technique that analyzes and sorts individual cells based on their fluorescence and light-scattering properties as they flow in a single stream past a laser.

Instrumentation/Principle:

1. Fluidics: Cells labeled with fluorochrome-tagged antibodies are suspended and passed single-file through a flow chamber (hydrodynamic focusing).
2. Optics (Laser): Each cell intercepts a laser beam, scattering light and exciting the fluorochromes.
   • Forward scatter (FSC): Indicates cell size.
   • Side scatter (SSC): Indicates internal complexity/granularity.
3. Detectors: Photomultiplier tubes detect scattered light and emitted fluorescence.
4. Sorting (FACS): Cells are given an electric charge and deflected into collection tubes based on their properties.

Applications:

• Immunophenotyping — counting CD4⁺ T cells in HIV/AIDS monitoring.
• Cell cycle and DNA content analysis.
• Detecting cell populations in leukemia/lymphoma diagnosis.
• Cell sorting for research.

Advantage: Rapid, multiparameter analysis of thousands of cells per second.

Prozone and Postzone Phenomena:

These phenomena describe how an imbalance in the ratio of antigen to antibody can prevent visible lattice formation, leading to false-negative results in serological tests.

Prozone Phenomenon (Antibody Excess):

• Occurs when there is a very high concentration of antibody relative to antigen.
• Each antigen molecule is coated by antibody, preventing cross-linking between antigen molecules.
• Result: No visible precipitation/agglutination despite antibodies being present → false negative.

Postzone Phenomenon (Antigen Excess):

• Occurs when there is a very high concentration of antigen relative to antibody.
• Antibody binding sites become saturated; only small soluble complexes form.
• Result: Little/no visible reaction → false negative.

Zone of Equivalence (Optimal):

• Between these zones lies the equivalence zone, where the antigen-antibody ratio is optimal, producing maximum lattice and visible reaction.

Clinical Significance:

• The prozone effect is notable in tests such as syphilis (VDRL) serology, where high antibody titers give false negatives.

How to Overcome:

1. Serial dilution of the serum (for prozone): Diluting reduces excess antibody, bringing the reaction into the equivalence zone → positive result appears.

2. Diluting/reducing antigen (for postzone) or retesting.

Conclusion: Awareness of these phenomena and appropriate dilution of samples is essential to avoid diagnostic errors.

Immunofluorescence vs. Immunohistochemistry:

Both techniques localize antigens in situ using specific antibodies; they differ mainly in the type of label and detection system.

Comparison Table:

Advantages of IF:

• High sensitivity; allows multiple antigens (multi-color) simultaneously.
• Good for detecting immune complex deposits (kidney, skin biopsies).

Limitations of IF:

• Requires a fluorescence microscope.
• Signal fades over time (photobleaching); slides are not permanent.
• Autofluorescence can interfere.

Advantages of IHC:

• Uses a standard light microscope — widely available.
• Produces permanent slides for archiving.
• Excellent tissue morphology with counterstains.

Limitations of IHC:

• Endogenous enzyme activity may cause background.
• Less suited to simultaneous multiplex detection.

Conclusion: IF is preferred for immune deposit detection and multiplexing, while IHC is favored in routine surgical pathology and cancer diagnostics due to permanent, morphology-preserving slides.