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#ELISA

18 public questions tagged with this topic.

Immunological Applications

ELISA is commonly used for:

Enzyme-linked immunosorbent assay revolutionized immunodiagnostics by converting antigen antibody binding into measurable enzymatic colorimetric signal without radioactivity, originally described by Eva Engvall and Peter Perlmann 1971. Polystyrene microtiter plate with high binding capacity via hydrophobic interaction adsorbs capture antibody coated overnight at alkaline pH 9.6 carbonate buffer 1 to 10 microgram per ml. Non-specific sites blocked with protein bovine serum albumin five percent, casein, or nonfat milk to prevent background adsorption through hydrophobic pockets. Sample containing antigen such as viral capsid HIV p24, cytokine IL-6, or hormone insulin incubated allowing specific capture, unbound material removed by repeated washes with phosphate buffered saline containing 0.05 percent Tween-20 disrupting weak interactions. Detection uses secondary antibody conjugated covalently to horseradish peroxidase or alkaline phosphatase via periodate oxidation forming Schiff base. Addition of chromogenic substrate tetramethylbenzidine TMB with hydrogen peroxide yields oxidation to blue diimine measured after sulfuric acid stopping as yellow product absorbance 450 nm proportional to analyte concentration via four-parameter logistic standard curve with dynamic range three logs.

Ref: Engvall & Perlman Immunochemistry 1971 ELISA; Alberts MBoC ELISA HRP TMB sandwich principle diagnostic.

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Immunological Applications

ELISA is commonly used for:

ELISA pioneered Engvall Perlmann 1971 exploits highly specific high-affinity antigen-antibody interaction KD nanomolar picomolar detecting analytes complex biological fluids serum urine CSF culture supernatant. Principle immobilization one component passive hydrophobic adsorption polystyrene 96-well high-binding blocking nonspecific sites BSA casein preventing background. Indirect sandwich formats most sensitive plate coated capture antibody monoclonal high-affinity against target antigen human chorionic gonadotropin hCG pregnancy marker HIV p24 capsid antigen cytokines IL-6 TNF-alpha allergen-specific IgE incubated specific immune complex formation governed mass action kinetics association rate 10^5 per M per s avidity bivalent enhancing stability. Washing PBS 0.05 percent Tween removing unbound proteins specific complex remains strong noncovalent hydrogen bonds van der Waals. Secondary antibody conjugated HRP enzyme 40 kDa via glutaraldehyde biotin-streptavidin amplification four biotin binding sites affinity 10^-15 M binding Fc primary antibody catalyzes oxidation chromogenic substrate TMB 3,3',5,5' presence H2O2 generating blue radical 650 nm sulfuric acid stop converts yellow diimine 450 nm measured spectrophotometrically microplate reader intensity directly proportional antigen concentration via 4PL standard curve interpolation recombinant standards. Sensitivity pg/mL dynamic range 3 logs high throughput screening hundreds samples daily low cost no radioisotope adaptability automation made ELISA cornerstone diagnostic infectious diseases autoimmunity allergy therapeutic monitoring.

Ref: Engvall and Perlmann 1971 ELISA Original J Immunol; NCBI Bookshelf ELISA Principles; Janeway Immunoassay ELISA Technique Variants Chapter 3.

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Blotting Techniques

A labeled antibody is required in:

Western blotting, synonymously called immunoblotting, relies fundamentally on specific antigen-antibody interaction for protein identification and quantification. Workflow involves SDS-PAGE separation, electrotransfer to membrane, blocking, incubation with unlabeled primary antibody against epitope, followed by labeled secondary antibody conjugated to horseradish peroxidase, alkaline phosphatase, fluorophore, or bioluminescent enzyme. Signal originates from label attached to antibody. Polymerase chain reaction amplifies DNA using oligonucleotide primers and does not involve antibodies. Southern and Northern blotting detect nucleic acids through labeled complementary probes, not immunoglobulins, highlighting unique requirement for labeled antibody in immunoblotting.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Flow cytometry, FACS, ELISA

Which ELISA format is best for measuring hormone levels like insulin?

Competitive ELISA is preferred for low molecular weight analytes present at low concentrations such as hormones insulin, triiodothyronine and steroids where sandwich assay fails due to single epitope and small size preventing dual antibody binding. Sample hormone competes with enzyme labeled hormone for limited antibody binding sites coated on plate. Signal inversely proportional to hormone level allows sensitive quantification in nanomolar range with small sample volume. Direct and indirect formats lack required sensitivity, sandwich requires two non overlapping epitopes unavailable on small hormones, making competitive format ideal.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Flow cytometry, FACS, ELISA

Which tag can increase chemiluminescence in ELISA?

Colorimetric ELISA uses HRP, ALP and beta galactosidase to produce colored products, but sensitivity limited. Chemiluminescence enhances detection using enzymes or proteins generating light in presence of substrate. Luciferase catalyzes luciferin oxidation producing photons detectable by luminometer, vastly amplified compared to absorbance. GFP is fluorescent not chemiluminescent, ALP can also produce chemiluminescence with appropriate dioxetane substrates but luciferase is archetypal bioluminescent tag increasing signal to noise ratio. Fusion of luciferase to antibody improves limit of detection for low abundance antigens.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Flow cytometry, FACS, ELISA

O-Phenyl diamine is used as a chromogenic substrate in?

ELISA relies on enzyme conjugated antibodies converting chromogenic substrates into colored products measurable spectrophotometrically. Horseradish peroxidase commonly uses TMB, ABTS and O phenylenediamine OPD producing orange brown soluble product read at 492 nm after acid stop. Alkaline phosphatase uses pNPP. OPD offers good sensitivity and stability for peroxidase detection. RIA uses radiolabeled isotopes measured by gamma counter, Southern and Western blotting may use chemiluminescence or colorimetric substrates but OPD is classic ELISA chromogen, not used in nucleic acid hybridization or radioactivity detection protocols.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Flow cytometry, FACS, ELISA

Which enhances ELISA sensitivity without compromising specificity?

Sensitivity in ELISA often enhanced via avidin biotin system without losing specificity. Primary antibody binds antigen, biotinylated secondary antibody binds primary, then streptavidin conjugated enzyme with four biotin binding sites attaches. Each secondary can carry multiple biotins, recruiting many enzyme molecules, amplifying signal dramatically. HRP and ALP alone give single enzyme per antibody. Direct detection lacks amplification steps. Biotinylation preserves antibody specificity because antigen recognition remains unchanged, while enzyme loading increases. This approach lowers detection limit for low abundance proteins and improves signal to noise ratio.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Flow cytometry, FACS, ELISA

ELISA detecting HIV antibodies is an example of:

ELISA formats differ by what is immobilized and detected. Direct ELISA coats antigen and uses enzyme labeled primary antibody. Sandwich ELISA uses two antibodies to trap antigen. Competitive ELISA uses competition for signal reduction. Indirect ELISA coats antigen, commonly viral lysate, patient serum antibodies bind, then enzyme conjugated anti human immunoglobulin detects bound human antibody. HIV screening detects anti HIV antibodies in patient serum against coated viral antigens, so uses unlabeled primary from patient plus labeled secondary antibody, characteristic of indirect ELISA ensuring amplification and sensitivity for serology.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Flow cytometry, FACS, ELISA

Which fluorochrome is self-fluorescent and does not require a substrate?

Fluorescent reporters differ markedly in substrate requirement and handling. Enzymes like horseradish peroxidase and alkaline phosphatase need exogenous chromogenic or chemiluminescent substrates TMB, pNPP, BCIP to produce detectable color or light signal. Luciferase requires luciferin and ATP to emit bioluminescence. Green Fluorescent Protein from jellyfish Aequorea victoria forms its internal chromophore autocatalytically through cyclization, requiring only excitation light near 488 nanometers to emit bright green light without added chemicals, fixation or permeabilization. This self contained fluorescence makes GFP ideal for live cell tracking.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Microscopy

Which ELISA is best to detect minor antigen concentration differences?

Competitive ELISA excels for low molecular weight haptens and subtle concentration variations because signal is inversely proportional to analyte and competition amplifies small differences. Limited antibody sites create steep competition curve around IC50, so slight rise in sample antigen displaces labeled competitor measurably. Sandwich formats depend on two epitopes and are better for larger proteins but suffer less resolution at low range because excess antigen increases signal linearly. Direct and indirect formats lack competition step and are less sensitive to minor changes. Hence for steroid hormones, toxins and drug monitoring, competitive design provides superior discrimination of minute differences.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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