Radioimmunoassay (RIA), classically used for quantifying hormones such as insulin and thyroxine, depends on which fundamental principle?

• A Sandwich binding of analyte between two labelled antibodies
• B Enzyme conversion of substrate generating a coloured product proportional to analyte
• C Competitive inhibition between radiolabelled and unlabelled antigen for limited antibody sites ✓
• D Agglutination of carrier particles by antigen-antibody cross-linking

Explanation

Classical RIA is a competitive assay: a fixed amount of radiolabelled antigen competes with unlabelled antigen from the sample for a limited quantity of specific antibody. Bound versus free radioactivity is then separated and measured, and the proportion bound falls inversely with the concentration of unknown antigen. Sandwich formats require excess antibody and two epitopes, enzyme labels define ELISA rather than RIA, and particle agglutination is a separate system, so these options misdescribe the assay.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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