ELISA (enzyme-linked immunosorbent assay) in the indirect format detects:

• A Antigen in the specimen by using enzyme-labelled specific antibody
• B Patient antibodies in serum by using known antigen coated on the solid phase ✓
• C Immune complexes using complement fixation
• D Antigen quantity using competitive inhibition of enzyme-labelled antigen

Explanation

In indirect (or indirect antibody) ELISA, known antigen is coated onto a solid phase (microtitre plate). Patient serum containing antibodies is added; if specific antibodies are present they bind the antigen. After washing, an enzyme-conjugated secondary antibody (anti-human IgG/IgM) is added, binds to the captured antibody, and the enzyme substrate produces a colour change proportional to antibody concentration. This format is used for serological diagnosis of infections (HIV, HCV, dengue IgM, toxoplasmosis). Sandwich (capture) ELISA detects antigen; competitive ELISA uses competition with enzyme-labeled antigen.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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