In ELISA, which type uses a labelled antibody directed against the analyte antibody (sandwich format) and is typically used to detect antibodies in patient serum for diagnosis of infectious disease?

• A Direct ELISA
• B Indirect ELISA ✓
• C Sandwich (capture) ELISA
• D Competitive ELISA

Explanation

Indirect ELISA is the most widely used format for detecting patient antibodies: the microtiter plate is coated with specific antigen, patient serum is added (if antibody is present it binds), and then an enzyme-labelled anti-human IgG secondary antibody detects the bound antibody, amplifying the signal. Direct ELISA uses a labelled primary antibody to detect antigen directly. Sandwich ELISA is used to quantitate antigens (e.g., HBsAg, p24 antigen). Competitive ELISA is used for small molecules and can detect either antibodies or antigens.

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

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