Conceptually, what is the main difference between a sandwich (direct) ELISA and an indirect ELISA?

• A. There is no conceptual difference; both names are interchangeable and both always detect patient antibodies directly using two enzyme-linked antibodies in sequence, one after the other, to amplify the color signal.
• B. Sandwich ELISA detects antigen in the patient sample captured between two antibodies, whereas indirect ELISA detects the patient’s primary antibody binding to a fixed antigen and uses an enzyme-linked secondary antibody for detection.
• C. Sandwich ELISA uses only a secondary antibody to capture patient antibodies on wells coated with enzyme, whereas indirect ELISA sandwiches antigens between two enzyme-linked antibodies to detect proteins by color change.
• D. Sandwich ELISA detects nucleic acids using antibody-enzyme complexes as labels, while indirect ELISA directly measures enzymatic degradation of substrate by patient-derived enzymes as a proxy for infection status.

Answer: (B)