ELISA design (direct antigen detection): If you perform a direct ELISA to detect polio virus antigen in a specimen, what should be coated onto the plastic microtiter wells as the capture layer?

Introduction / Context:
Enzyme-linked immunosorbent assays (ELISA) rely on specific antigen-antibody interactions. Proper plate coating is essential to capture the target and achieve specificity and sensitivity.

Given Data / Assumptions:

• Goal: detect polio virus antigen present in the test sample.
• Direct antigen-detection ELISA uses an immobilized capture antibody.
• Signal is generated by an enzyme-labeled antibody or detection reagent binding the captured antigen.

Concept / Approach:
For direct detection of antigen, wells are coated with a specific capture antibody that binds the target antigen from the sample. Patient serum is not a capture reagent in this design. Antigen-coated plates are used for antibody detection assays, not antigen detection.

Step-by-Step Solution:
Define the assay objective: detect viral antigen. Choose a capture layer specific to that antigen: anti-polio antibody. Apply specimen; antigen binds to immobilized antibody. Add enzyme-conjugated detection antibody to generate measurable signal.

Verification / Alternative check:
Compare against indirect ELISA (antibody detection) where antigen is coated and patient antibodies are probed. Here, the setup is the reverse because the target is antigen.

Why Other Options Are Wrong:

• Patient serum: Contains unknowns; not a defined capture reagent.
• Polio capsid protein: Used to detect anti-polio antibodies, not to capture antigen.
• Colored substrate: Added after enzyme binding, not as a coating.
• Blocking protein only: Used to prevent nonspecific binding, not to capture target.

Common Pitfalls:
Confusing antigen-capture ELISA with antibody-detection formats; misplacing blocking and detection steps.

Final Answer:
Anti-polio antibody.