Enzyme-Linked Immunosorbent Assay (ELISA): Indirect & Sandwich Microplate Assays
🔬 Principle & Molecular Cascades
ELISA (Enzyme-Linked Immunosorbent Assay) was invented independently in 1971 by Peter Perlmann and Eva Engvall in Sweden and Anton Schuurs and Bauke van Weemen in the Netherlands as a non-radioactive alternative to Radioimmunoassay (RIA).
ELISA relies on two essential biochemical properties:
1. Solid-phase adsorption: Polystyrene or polyvinyl chloride 96-well microplates possess hydrophobic surfaces that passively adsorb proteins through hydrophobic and charge interactions.
2. Enzyme amplification: An antibody or antigen is covalently conjugated to an enzyme (Horseradish Peroxidase HRP or Alkaline Phosphatase AP) that catalytically converts a colorless substrate into a chromogenic soluble product. A single bound enzyme molecule turnover converts millions of substrate molecules per minute, granting ELISA femtomolar analytical sensitivity ($10^{-15},text{M}$).
- Indirect ELISA Architecture: Solid Phase → Coated Target Antigen → Blocking Unoccupied Sites (BSA) → Primary Antibody (Patient Serum IgG) → Wash → Enzyme-Conjugated Secondary Antibody (Goat Anti-Human IgG-HRP) → Wash → Chromogenic Substrate (TMB + $text{H}_2text{O}_2$) → Stop Solution ($2,text{N H}_2text{SO}_4$) → Read at $450,text{nm}$. Signal is directly proportional to serum antibody titer.
- Sandwich ELISA Architecture: Solid Phase → Immobilized Capture Antibody → Blocking → Analyte Antigen Sample → Wash → Enzyme-Conjugated Detection Antibody → Wash → TMB Substrate → Read at $450,text{nm}$. The target antigen is “sandwiched” between two distinct antibodies that recognize non-overlapping epitopes. Offers the highest analytical specificity.
📋 Step-by-Step Laboratory Workflow (Sandwich ELISA)
- Coating Microplate Wells: Dilute capture monoclonal antibody to $2-5,mutext{g/mL}$ in carbonate-bicarbonate coating buffer ($0.05,text{M}$, pH 9.6). Dispense $100,mutext{L}$ into each well of a high-binding 96-well polystyrene plate. Seal and incubate at $4^circtext{C}$ overnight (or $37^circtext{C}$ for 2 hours).
- Washing: Aspirate coating buffer. Wash each well 3 times with $300,mutext{L}$ of Wash Buffer (PBST: Phosphate Buffered Saline containing $0.05%$ Tween-20). Invert and firmly slap the plate onto paper towels to remove residual droplets.
- Blocking Non-Specific Sites: Add $200,mutext{L}$ of blocking solution ($1-3%$ Bovine Serum Albumin BSA or $5%$ non-fat dry milk in PBST) to prevent non-specific binding of subsequent reactants. Incubate at room temperature for 1 hour; wash $3times$.
- Sample & Standard Incubation: Add $100,mutext{L}$ of serial standard dilutions and unknown serum samples in duplicate. Incubate at $37^circtext{C}$ for 1 hour; wash $4times$ with PBST.
- Enzyme Conjugate Addition: Add $100,mutext{L}$ of Horseradish Peroxidase-conjugated detection antibody (diluted $1:5000$ in blocking buffer). Incubate at room temperature for 1 hour in the dark; wash $5times$ thoroughly.
- Chromogenic Development: Add $100,mutext{L}$ of freshly prepared Tetramethylbenzidine (TMB) substrate solution containing hydrogen peroxide ($text{H}_2text{O}_2$). Incubate at room temperature for 15-20 minutes in the dark until a blue color develops.
- Reaction Stopping & Absorbance Reading: Add $50,mutext{L}$ of $2,text{N H}_2text{SO}_4$ stop solution. The sulfuric acid protonates the oxidized TMB diamine, instantly halting the peroxidase reaction and shifting the blue product to a bright, stable yellow color. Read optical density (OD) immediately on an ELISA microplate spectrophotometer at $lambda = 450,text{nm}$ (reference filter $620-650,text{nm}$).
