Immunoassays

Introduction

Immunoassay: analytical technique using antibody-antigen binding for detection and quantification. Principle: antibody's high specificity and affinity enables selective measurement of target molecule. Sensitivity: femtomolar to nanomolar (depending on format). Applications: clinical diagnostics (dominant platform), research, food safety, environmental testing. Volume: billions of immunoassay tests performed annually worldwide. Market: $25+ billion (clinical immunoassay market).

"The immunoassay exploits nature's most sophisticated recognition system,the antibody. With binding affinities that rival the best synthetic receptors and specificity that distinguishes molecules differing by a single atom, antibodies are the ideal biosensing element." -- Immunochemist

Antibody-Antigen Interaction

Antibody Structure

IgG: Y-shaped protein (~150 kDa), two identical heavy chains + two light chains. Variable region: antigen binding site (CDRs, complementarity-determining regions). Constant region: Fc portion (determines antibody class, effector functions). Valency: IgG has 2 binding sites, IgM has 10. Diversity: >10¹⁰ different antibody specificities possible.

Binding Kinetics

Ab + Ag ⇌ Ab-AgKa (association rate): 10⁵ - 10⁸ M⁻¹s⁻¹Kd (dissociation rate): 10⁻¹ - 10⁻⁴ s⁻¹KD (equilibrium dissociation constant): Kd/KaTypical KD: 10⁻⁷ - 10⁻¹² MLower KD = higher affinity = better for immunoassay

Affinity vs. Avidity

Affinity: strength of single binding site interaction. Avidity: overall strength with multiple binding sites (IgM > IgG). Effective affinity: enhanced by multivalent binding (surface immobilization). Assay impact: higher affinity = lower detection limit, better specificity.

Cross-Reactivity

Definition: antibody binds structurally similar molecules. Cause: epitope sharing between related molecules. Example: cortisol antibody cross-reacts with cortisone. Impact: false positives, inaccurate quantification. Mitigation: select highly specific antibodies, validate against related compounds.

ELISA

Enzyme-Linked Immunosorbent Assay

Principle: antibody linked to enzyme, enzymatic reaction produces measurable signal. Format: 96-well microplate (standard). Detection: colorimetric (most common), fluorescence, or chemiluminescence. Sensitivity: pg/mL to ng/mL range. Time: 2-5 hours. Cost: $200-500 per kit (96 wells). Standard: most widely used immunoassay format in research.

ELISA Formats

Sandwich ELISA Protocol

Coat: capture antibody on plate (overnight, 4°C). Block: BSA or casein (prevent non-specific binding). Sample: add sample (antigen binds capture antibody). Detection: enzyme-labeled detection antibody binds second epitope. Substrate: TMB (for HRP) or pNPP (for AP). Read: spectrophotometer at 450 nm (TMB) or 405 nm (pNPP). Standard curve: serial dilution of known antigen (quantification).

Enzymes and Substrates

HRP (horseradish peroxidase): most common, TMB substrate (blue → yellow with acid). AP (alkaline phosphatase): pNPP substrate (yellow). Sensitivity: HRP generally more sensitive. Stability: AP more stable (longer shelf life). Signal amplification: each enzyme converts thousands of substrate molecules.

Lateral Flow Immunoassay

Design

Strip: nitrocellulose membrane with immobilized antibodies. Sample pad: receives sample (blood, urine, saliva, nasal swab). Conjugate pad: gold nanoparticle-labeled antibodies. Test line: capture antibody (positive = colored line). Control line: anti-species antibody (must always appear for valid test). Result: 5-15 minutes (rapid).

Applications

Pregnancy test: detect hCG in urine (first lateral flow, 1988). COVID-19 rapid test: detect SARS-CoV-2 antigen. HIV: rapid antibody test (finger-prick blood). Malaria: detect parasite antigens. Drug testing: workplace drug screening. Food safety: pathogen detection.

Performance

Sensitivity: lower than ELISA (~ng/mL vs. pg/mL). Specificity: good (depends on antibody quality). Speed: fastest immunoassay format (minutes). Cost: $1-20 per test (cheapest format). Quantification: semi-quantitative with reader, qualitative by eye. Trade-off: speed and simplicity vs. sensitivity.

Reader Technology

Visual: yes/no interpretation (qualitative). Lateral flow readers: quantify band intensity. Smartphone: camera-based quantification apps. Fluorescent labels: improved sensitivity with UV reader. Electrochemical: integrated electrical detection. Trend: moving from qualitative to quantitative.

Radioimmunoassay

Principle

Label: radioactive isotope (I-125, H-3, C-14). Competition: labeled antigen competes with sample antigen for antibody binding. Measurement: gamma or beta counter measures radioactivity. Sensitivity: pg/mL (historically most sensitive immunoassay). Nobel Prize: Rosalyn Yalow (1977, developed RIA for insulin).

Decline in Use

Radiation safety: handling radioactive materials requires licensing. Waste disposal: radioactive waste expensive to manage. Shelf life: radioactive labels decay (short usable period). Replacement: chemiluminescence and ECLIA offer similar sensitivity without radiation. Remaining use: specialized hormone assays (decreasing).

Fluorescence Immunoassays

Fluorescence Labels

Fluorescein (FITC): green emission, standard for microscopy. PE (phycoerythrin): brilliant red, used in flow cytometry. Alexa Fluor: bright, photostable (multiple colors available). Quantum dots: tunable emission, excellent for multiplexing. Europium chelates: time-resolved (DELFIA, eliminates autofluorescence).

Time-Resolved Fluorescence (TRF)

Principle: lanthanide labels (Eu, Tb) have long fluorescence lifetime (~1 ms). Detection: measure after autofluorescence decays (~10 ns). Advantage: virtually zero background (exceptional signal-to-noise). Platform: DELFIA (PerkinElmer), HTRF (Cisbio). Sensitivity: 0.1 pg/mL achievable. Application: high-throughput drug screening, clinical assays.

FRET-Based Immunoassays

Principle: donor and acceptor fluorophores on different antibodies. Binding: brings donor/acceptor close → FRET signal. Advantage: homogeneous (no wash steps needed). Speed: minutes (mix and read). Application: high-throughput screening. Platform: HTRF, Alpha (PerkinElmer).

Chemiluminescence Immunoassays

Principle

Label: chemiluminescent molecule or enzyme producing light. Signal: chemical reaction generates photons (no excitation light needed). Advantage: very low background (no excitation → no autofluorescence). Sensitivity: femtomolar (best non-isotopic sensitivity). Dominant: clinical chemistry platform (>80% of hospital immunoassays).

Electrochemiluminescence (ECLIA)

Label: ruthenium complex (Ru(bpy)3²⁺). Trigger: electrical stimulation at electrode surface. Platform: Roche Elecsys/cobas (dominant clinical platform). Advantage: controlled timing, low background, wide dynamic range. Throughput: >200 tests/hour per module. Application: thyroid (TSH, T4), cardiac (troponin), tumor markers (PSA), infectious disease (HIV, hepatitis).

Acridinium Ester

Mechanism: flash chemiluminescence triggered by H2O2 + NaOH. Platform: Abbott Architect, Siemens ADVIA Centaur. Advantage: rapid signal generation (seconds). Application: similar clinical menu to ECLIA. Market: major clinical immunoassay platform.

Clinical Analyzers

Random access: any test on any sample (no batching). Throughput: 200-400+ tests/hour. Menu: 50-100+ different analytes. Automation: sample handling, reagent management, result reporting. Integration: connected to LIS (laboratory information system). Dominant platforms: Roche cobas, Abbott Alinity, Siemens Atellica, Beckman Coulter DxI.

Multiplex Immunoassays

Bead-Based (Luminex)

Principle: different colored beads coated with different capture antibodies. Detection: PE-labeled detection antibody (fluorescence). Identification: bead color identifies analyte. Quantification: PE fluorescence proportional to analyte concentration. Capacity: up to 500 analytes per well. Volume: 25-50 µL sample per well. Application: cytokine panels, biomarker discovery.

Planar Arrays

Meso Scale Discovery (MSD): electrochemiluminescence on multi-spot plates. Quanterix Simoa: single molecule counting (ultra-sensitive). Olink: proximity extension assay (antibody pairs + DNA amplification). Advantage: high sensitivity, multiplexing. Application: biomarker panels, clinical trials.

Single-Molecule Counting (Simoa)

Principle: capture analyte on magnetic beads → label → isolate individual beads in femtoliter wells. Detection: single molecule detection (digital counting). Sensitivity: femtomolar (1000x better than standard ELISA). Application: detect extremely low-abundance biomarkers (neurofilament light, tau in blood). Impact: enabled blood-based Alzheimer's biomarkers.

Antibody Production

Polyclonal Antibodies

Production: immunize animal (rabbit, goat, mouse) with antigen. Serum: collect blood, purify IgG. Mixture: contains antibodies against multiple epitopes. Advantage: high avidity, tolerant of antigen variation. Disadvantage: batch-to-batch variation, finite supply. Cost: $500-5,000 (custom production). Timeline: 2-3 months.

Monoclonal Antibodies

Production: hybridoma technology (Kohler and Milstein, 1975). Process: fuse antibody-producing B cell with myeloma cell. Result: immortal cell line producing single antibody specificity. Advantage: unlimited supply, batch consistency, single epitope. Disadvantage: expensive production, may not recognize denatured antigen. Cost: $5,000-50,000 (custom). Timeline: 4-6 months.

Recombinant Antibodies

Production: express antibody genes in bacteria, yeast, or mammalian cells. Formats: scFv, Fab, nanobody (VHH). Advantage: animal-free, engineered properties, rapid production. Library screening: phage display, yeast display (in vitro selection). Application: research, therapeutic development. Trend: replacing hybridoma technology.

Antibody Validation

Critical: antibody specificity must be verified. Methods: knockout validation, orthogonal techniques, multiple antibodies. Problem: many commercial antibodies not adequately validated. Impact: irreproducible results, wasted resources. Initiative: antibody validation consortia promoting standards.

Assay Validation

Key Parameters

Sensitivity (LOD): lowest detectable concentration. Quantification limit (LOQ): lowest accurately quantifiable concentration. Precision: reproducibility (intra-assay CV <10%, inter-assay CV <15%). Accuracy: recovery of known spike (80-120% acceptable). Linearity: proportional response over concentration range. Specificity: no cross-reactivity with related molecules.

Standard Curve

Serial dilution: known antigen concentrations (7-10 points). Fitting: 4-parameter logistic (4PL) curve fit (standard for immunoassays). Dynamic range: typically 2-3 logs (100-fold range). Back-calculation: sample concentration from curve. Acceptance: R² > 0.99, standards within ±20% of nominal.

Quality Control

Controls: low, medium, high concentration QC samples. Acceptance: within ±2 SD of established mean. Levey-Jennings chart: track QC performance over time. Westgard rules: statistical rules for run acceptance/rejection. Proficiency testing: external quality assessment (CAP, EQAS).

Interference and Troubleshooting

Common Interferences

HAMA (human anti-mouse antibodies): bridge capture and detection antibodies (false positive). Biotin: high-dose biotin supplements interfere with streptavidin-based assays. Heterophilic antibodies: non-specific bridging. Hemolysis: released hemoglobin absorbs light. Lipemia: turbidity affects optical reading. Hook effect: extremely high analyte saturates both antibodies (falsely low result).

High-Dose Hook Effect

Mechanism: excess antigen saturates both capture and detection antibodies separately. Result: no sandwich formed → signal decreases at very high concentrations. Clinical risk: dangerously low result reported for extremely high analyte (e.g., hCG in molar pregnancy). Detection: test neat and diluted sample (diluted should give lower result; if higher → hook). Prevention: two-step assay format, sample dilution protocol.

Matrix Effects

Definition: sample components other than analyte affect assay performance. Serum vs. plasma: different results possible (fibrinogen in plasma). Dilution linearity: serial dilutions should give proportional results. Spike recovery: add known amount, verify expected recovery. Mitigation: matrix-matched calibrators, sample dilution.

Clinical Applications

Endocrinology

Thyroid: TSH, free T4, free T3. Reproductive: hCG (pregnancy), estradiol, progesterone, testosterone. Adrenal: cortisol, ACTH, aldosterone. Growth: IGF-1, growth hormone. Diabetes: insulin, C-peptide. Volume: endocrine testing = largest immunoassay segment.

Infectious Disease

HIV: screening (4th gen: p24 antigen + antibody). Hepatitis: HBsAg, anti-HCV, viral load. COVID-19: rapid antigen test (lateral flow), antibody testing. Syphilis: RPR screening, treponemal confirmation. CMV, EBV: IgM (acute), IgG (past infection). Approach: screen high-sensitivity, confirm high-specificity.

Tumor Markers

PSA: prostate cancer screening and monitoring. CA-125: ovarian cancer monitoring. CEA: colorectal cancer recurrence. AFP: hepatocellular carcinoma, testicular cancer. CA 19-9: pancreatic cancer. Limitation: most tumor markers lack sensitivity/specificity for screening (monitoring preferred).

Cardiac Markers

High-sensitivity troponin: myocardial infarction diagnosis (standard of care). BNP/NT-proBNP: heart failure diagnosis and monitoring. D-dimer: pulmonary embolism screening. CK-MB: myocardial infarction (largely replaced by troponin). Impact: rapid cardiac marker testing enables faster ER decisions.

Autoimmune

ANA: anti-nuclear antibody (screening for lupus, autoimmune disease). Anti-dsDNA: specific for SLE. RF: rheumatoid factor (rheumatoid arthritis). Anti-CCP: specific for rheumatoid arthritis. Thyroid antibodies: TPO, TG antibodies (Hashimoto's, Graves'). Panel approach: multiple autoantibodies tested simultaneously.

References

• Wild, D. (Ed.). "The Immunoassay Handbook." Elsevier, 4th ed., 2013.
• Engvall, E., and Perlmann, P. "Enzyme-Linked Immunosorbent Assay (ELISA)." The Journal of Immunology, vol. 109, no. 1, 1972, pp. 129-135.
• Yalow, R. S., and Berson, S. A. "Immunoassay of Endogenous Plasma Insulin in Man." The Journal of Clinical Investigation, vol. 39, no. 7, 1960, pp. 1157-1175.
• Kohler, G., and Milstein, C. "Continuous Cultures of Fused Cells Secreting Antibody of Predefined Specificity." Nature, vol. 256, 1975, pp. 495-497.
• Rissin, D. M., Kan, C. W., Campbell, T. G., et al. "Single-Molecule Enzyme-Linked Immunosorbent Assay Detects Serum Proteins at Subfemtomolar Concentrations." Nature Biotechnology, vol. 28, 2010, pp. 595-599.