ELISA: Basics - Antibody Adventures
- ELISA: Enzyme-Linked Immunosorbent Assay.
- Core Principle: Specific antigen (Ag) binds to an antibody (Ab) - an "antibody adventure".
- Detection: An enzyme, linked to an Ab (or Ag), reacts with a substrate. This produces a measurable signal (e.g., color change), proportional to the Ag/Ab amount.
- Key Function: Detects and quantifies substances like peptides, proteins (e.g., hormones, viral antigens), and antibodies.
- Solid-phase assay: One component (Ag or Ab) is immobilized on a microplate well.
⭐ ELISA is a primary screening test for HIV, detecting anti-HIV antibodies in patient serum or plasma.
ELISA: Toolkit - Essential Ingredients
- Microtiter Plate: Polystyrene, 96-well; solid phase for adsorption.
- Antigen (Ag) / Antibody (Ab): Known biomolecule for coating or detection.
- Sample: Patient serum/fluid containing target analyte (Ag or Ab).
- Enzyme-Conjugated Antibody: Secondary Ab linked to enzyme (HRP, AP).
- Chromogenic Substrate: Reacts with enzyme, produces color (TMB, pNPP).
- Stopping Solution: Halts reaction, stabilizes color (e.g., $H_2SO_4$).
- Wash Buffers: Remove unbound reagents (e.g., PBS-Tween).
⭐ Horseradish Peroxidase (HRP) is a common ELISA enzyme due to its stability and high catalytic activity.
ELISA: Flavours - Spot The Difference
Key ELISA types-Direct, Indirect, Sandwich, Competitive-vary in component arrangement, influencing detection, sensitivity, and specific applications.
| ELISA Type | Well Coating | Detects (Sample) | Enzyme On | Signal vs [Analyte] | Key Use / Feature |
|---|---|---|---|---|---|
| Direct | Antigen | Antigen | Primary Ab | Proportional | Basic Ag detection; rapid; lower sensitivity |
| Indirect | Antigen | Antibody | Secondary Ab | Proportional | Ab screening (e.g., HIV); higher sensitivity (amplification) |
| Sandwich | Capture Ab | Antigen | Detection Ab | Proportional | Ag quantification (e.g., hormones, cytokines); high specificity |
| Competitive | Ag (or Ab) | Ag or Ab | Labeled competitor | Inverse | Small analytes (e.g., haptens); signal inversely proportional to analyte |
ELISA: Steps - The Assay Dance
⭐ The amount of color produced is directly proportional to the amount of specific antibody/antigen in the sample.
ELISA: Uses & IQ - Smart Assay
- Uses (Diagnostics & Research):
- Detecting antigens (e.g., viral proteins like p24 in HIV, HBsAg).
- Detecting antibodies (e.g., HIV, HCV, H. pylori, autoimmune diseases).
- Hormone assays (e.g., hCG, TSH, insulin).
- Drug screening & toxicology.
- Cancer marker detection (e.g., PSA, CEA).
- Interpretation & Quality (IQ):
- Qualitative: Presence/absence of Ag/Ab (e.g., positive/negative result).
- Quantitative: Concentration of Ag/Ab (correlates with signal intensity).
- High sensitivity & specificity.
- Relatively inexpensive & automatable.
⭐ ELISA is widely used for initial HIV screening due to its high sensitivity, followed by Western blot for confirmation due to its higher specificity.
- Limitations:
- Potential for false positives/negatives.
- Cross-reactivity can occur.
- Window period limitations for infectious diseases (e.g., HIV).
High‑Yield Points - ⚡ Biggest Takeaways
- Principle: Enzyme-linked immunosorbent assay detects antigens or antibodies via specific binding.
- Mechanism: An enzyme converts a substrate into a measurable colored product.
- Indirect ELISA: Detects antibodies (e.g., HIV antibodies); highly sensitive.
- Sandwich ELISA: Detects antigens (e.g., HBsAg, p24); uses two antibodies, very specific.
- Competitive ELISA: Signal inversely proportional to analyte; for small molecules/haptens.
- Key uses: Screening test for HIV, hormone assays, detecting viral antigens.
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