7 public questions tagged with this topic.
ELISA (Enzyme-Linked Immunosorbent Assay) is used to detect specific proteins or antigens in a sample.
Ref: NCERT Class 12 Biology Chapter 10: Biotechnology and its Applications Medical Applications - Molecular Diagnosis - PCR ELISA
Direct ELISA is simplest format where antigen from sample is adsorbed directly to polystyrene plate through passive hydrophobic interactions. After blocking, enzyme conjugated primary antibody specific to antigen is added, binding directly without secondary antibody. Substrate addition generates signal directly proportional to antigen present. It detects antigen not antibody, unlike indirect ELISA which detects antibodies. Only one antibody layer reduces steps but lowers amplification and specificity compared to sandwich formats. Its utility lies in measuring purified antigens or screening antibody specificity with enzyme labeled primary antibody.
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.
Competitive ELISA is advantageous for small molecules, haptens and subtle structural differences that cannot accommodate two antibodies simultaneously in sandwich format. One antigenic site allows competition between labeled reference and sample antigen for limited antibody. Minor variations in epitope structure alter competitive displacement curves, enabling discrimination of analogs, drug metabolites, hormone isoforms and pesticide residues. Antibody subclass identification needs isotype specific secondaries, protein folding requires biophysical methods, cell counting needs cytometry. Sensitive competition reveals small antigenic differences undetectable by direct binding alone.
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.
Sandwich ELISA provides high specificity for antigen quantification. Capture antibody specific to target is immobilized on plate, blocking reduces nonspecific binding. Antigen present in sample binds capture antibody and is retained. A second detection antibody recognizing different epitope on same antigen is added, sandwiched between two antibodies. Enzyme conjugated secondary antibody then generates colorimetric signal proportional to antigen. Direct ELISA coats antigen directly, indirect detects antibodies using labeled secondary, competitive relies on competition. Sandwich format is preferred for proteins like cytokines.
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.
Competitive ELISA measures small antigens where two antibodies cannot bind simultaneously. Wells are coated with reference antigen, sample antigen competes with labeled antigen for limited antibody binding sites. If sample contains abundant target, it occupies antibodies and prevents binding to plate antigen, so labeled complex is washed away and little color develops after substrate addition. Thus strong color indicates antibodies bound plate antigen because no competitor was present, meaning sample was negative for target analyte. Signal is inversely proportional to antigen concentration.
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.
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.
Radioimmunoassay achieves exceptional sensitivity through detection of gamma emissions from iodine-125 labeled antigen or antibody. Radioactive decay counting by gamma counters can resolve extremely low picomolar to femtomolar analyte concentrations, surpassing typical enzyme chromogenic limits where product must accumulate to visible levels. High affinity antibodies combined with competitive displacement allow quantification of hormones, thyroid hormones and small metabolites. While Western blot, immunodiffusion and standard ELISA are valuable for larger amounts or qualitative confirmation, they require higher analyte mass and generate higher background, making RIA historically gold standard for trace quantification despite safety constraints.
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.
