Unit 7: Immunological assays

Immunological assays exploit the highly specific, non-covalent binding between an antigen and its complementary antibody to detect and quantify either partner in a complex mixture. This unit centres on the Enzyme-Linked Immunosorbent Assay (ELISA), first described by Engvall and Perlmann and by Van Weemen and Schuurs (independently, 1971), which converts the invisible antigen–antibody binding event into a measurable colour signal by coupling one reactant to an enzyme.

I. Antigen–Antibody Interaction: The Governing Principle

An antibody (immunoglobulin) recognises a defined region of an antigen through a stereochemically complementary binding site, and the strength and specificity of this pairing is what every immunoassay reads out.

• Epitope and paratope: The epitope is the small antigenic determinant (typically 5–15 amino acids or a sugar sequence); the paratope is the antibody's antigen-binding site formed by the variable regions of heavy and light chains (V_H and V_L).
• Bond types: Binding is reversible and non-covalent — hydrogen bonds, electrostatic (ionic) forces, van der Waals interactions and hydrophobic contacts acting together over a close-fit surface.
• Affinity vs. avidity: Affinity is the strength of a single epitope–paratope bond (expressed as the equilibrium constant K_a, in M⁻¹); avidity is the combined strength of all bonds when a multivalent antibody (e.g. IgG with 2 sites, IgM with 10) binds a multivalent antigen.
• Specificity: A well-raised antibody discriminates between closely related epitopes; loss of this gives cross-reactivity, the main source of false positives.
• Immobilisation and detection: Because the interaction itself is silent, assays fix one partner to a solid phase and tag the other with a reporter — an enzyme in ELISA — so bound complexes can be separated by washing and then visualised.

II. Enzyme-Linked Immunosorbent Assay (ELISA)

A solid-phase assay converting antigen–antibody binding into a colorimetric signal

A. Purpose and Principle

ELISA detects or quantifies an antigen or antibody by immobilising one reactant on a plastic surface, capturing its partner, and reporting the bound complex through an enzyme-catalysed colour change proportional to the amount bound.

• Solid phase: A 96-well polystyrene microtiter plate adsorbs proteins passively through hydrophobic interactions, providing a washable surface that separates bound from free reagents.
• Reporter enzyme: The detecting antibody carries a covalently linked enzyme — most commonly horseradish peroxidase (HRP) or alkaline phosphatase (ALP) — chosen for high turnover so few molecules produce a strong signal.
• Substrate conversion: Adding the enzyme's substrate yields a coloured product; signal intensity is read as optical density (OD).
  • HRP + TMB (3,3′,5,5′-tetramethylbenzidine): blue, read at 370/652 nm, turning yellow and read at 450 nm after acid stop.
  • HRP + OPD (o-phenylenediamine): orange, read at 492 nm.
  • ALP + pNPP (p-nitrophenyl phosphate): yellow, read at 405 nm.
• Quantification: OD is measured on a plate reader (spectrophotometer); a standard curve of known antigen concentrations relates OD to unknown concentration, following the Beer–Lambert relation:

A = ε · c · l
A = absorbance (OD, unitless)
ε = molar absorptivity of the coloured product (M⁻¹ cm⁻¹)
c = concentration of product (M)
l = path length of the well (cm)

B. Demonstration of Antigen–Antibody Interaction by ELISA Method

The interaction is demonstrated by building a stepwise "immunosandwich" on the plate and washing away everything that has not bound, so that the final colour appears only where specific antigen–antibody complexes have formed.

• Common reagents and buffers:

  • Coating buffer: carbonate–bicarbonate, pH 9.6 — deprotonates protein surfaces to promote adsorption.
  • Blocking agent: 1–5% bovine serum albumin (BSA) or skim-milk powder — saturates unoccupied plastic to prevent non-specific sticking.
  • Wash buffer: phosphate-buffered saline with 0.05% Tween-20 (PBS-T) — removes unbound reactants; the detergent disrupts weak, non-specific attachments while sparing specific bonds.
  • Stop solution: dilute H₂SO₄ (for TMB) — halts the enzyme reaction to fix OD at a defined time.

• Generalised procedure (indirect format shown):

  1. Coat: dispense antigen in coating buffer, incubate (often overnight at 4 °C) so it adsorbs to the wells.
  2. Block: add BSA to cover remaining plastic.
  3. Primary antibody: add test serum; specific antibody binds the coated antigen.
  4. Wash: PBS-T removes unbound antibody.
  5. Conjugate: add enzyme-labelled anti-species (secondary) antibody, which binds the primary.
  6. Wash again.
  7. Substrate: add TMB; bound enzyme generates colour.
  8. Stop and read OD at 450 nm.

• Controls that prove specificity:

  • Blank well: substrate only — establishes background OD.
  • Negative control: no primary antibody (or irrelevant serum) — colour here signals non-specific binding.
  • Positive control: known reactive sample — confirms reagents are working.

• Reading the demonstration: a sample OD significantly above the negative control and background confirms a genuine antigen–antibody interaction; the magnitude of OD reflects the quantity of bound complex.

The same principle is realised through four formats, distinguished by which reactant is coated and how detection is arranged.

1. Direct vs. Indirect ELISA (antibody detection contrast)

• Direct ELISA: Coated antigen is detected by a single enzyme-labelled primary antibody.
  • Advantage: fewer steps, no cross-reactivity from a secondary antibody.
  • Limitation: every primary antibody must be individually conjugated; lower signal amplification.
• Indirect ELISA: Coated antigen is bound by an unlabelled primary antibody, then detected by an enzyme-labelled secondary antibody directed against the primary's species.
  • Advantage: one labelled secondary serves many primaries; multiple secondaries per primary amplify the signal.
  • Use: the standard format for detecting serum antibodies, e.g. screening for anti-HIV or anti-SARS-CoV-2 antibodies.

2. Sandwich vs. Competitive ELISA (antigen detection contrast)

• Sandwich ELISA: A capture antibody is coated first; it traps the antigen, which is then detected by a labelled detection antibody binding a second epitope.
  • Requirement: the antigen must be large enough to present two distinct epitopes.
  • Strength: high specificity (two antibodies must both recognise the antigen); ideal for measuring hormones and cytokines such as HCG or interleukins.
• Competitive (inhibition) ELISA: Sample antigen competes with a fixed amount of labelled antigen (or labelled antibody) for limited binding sites.
  • Signal relationship: OD is inversely proportional to sample antigen — more antigen means less label bound, so less colour.
  • Use: small haptens with a single epitope, e.g. drug or pesticide residues.

C. Applications and Limitations

ELISA's specificity, quantitative output and adaptability to 96-well throughput make it the workhorse of clinical and research immunoassays, but its performance depends on antibody quality and careful technique.

• Diagnostic applications:
  • Infectious disease serology: detecting antibodies to HIV, hepatitis B surface antigen (HBsAg), dengue and SARS-CoV-2.
  • Hormone and marker assays: pregnancy testing (HCG), thyroid and fertility hormones, cardiac and tumour markers.
  • Allergy and autoimmunity: measuring allergen-specific IgE and autoantibodies.
• Research and industrial uses: cytokine profiling, quantifying recombinant protein yields, food-safety screening for allergens and toxins (e.g. aflatoxin).
• Strengths:
  • Sensitivity: detects antigen in the nanogram-to-picogram per millilitre range owing to enzymatic amplification.
  • Quantitation: OD values fitted to a standard curve give concentrations, not just yes/no results.
  • Safety and cost: uses non-radioactive reporters, unlike the earlier radioimmunoassay it largely replaced.
• Limitations:
  • Cross-reactivity: structurally similar antigens can bind the antibody, giving false positives.
  • Antibody dependence: results are only as good as antibody affinity and specificity; poor antibodies raise background.
  • Procedural sensitivity: inadequate washing leaves non-specific colour; incomplete blocking or edge-effects in outer wells distort OD.
  • Enzyme instability: conjugate activity declines with age and improper storage, drifting the signal.

III. Sources of Error and Assay Validation

Reliable demonstration of the antigen–antibody interaction depends on distinguishing true binding from artefact, which is why every plate carries controls and defined acceptance criteria.

A. Non-Specific Binding and Background

Unwanted adsorption of detection reagents to the plastic or to blocking protein mimics a positive result and must be suppressed.

• Cause: hydrophobic patches on antibodies stick to under-blocked wells; high conjugate concentration worsens it.
• Control: thorough blocking (BSA/milk) and detergent washes (PBS-T) reduce it; the negative control quantifies whatever remains.

B. Cut-off Value and Interpretation

A threshold OD separates positive from negative samples, converting a continuous reading into a diagnostic call.

• Cut-off: commonly set as mean OD of negatives plus a multiple of their standard deviation (e.g. mean + 3 SD).
• Sample-to-cut-off (S/CO) ratio: the sample OD divided by the cut-off; values ≥ 1 are reported reactive.
• Trade-off: a low cut-off raises sensitivity but lowers specificity, increasing false positives — the balance is set by the clinical purpose of the assay.