How is ELISA used to quantify cytokines in patient samples?

• A. Western blot
• B. Agglutination
• C. Sandwich or capture ELISA uses specific antibodies and a standard curve to quantify cytokine concentration in serum or plasma
• D. Flow cytometry

Answer: (C)

Measuring cytokines in patient samples with ELISA relies on a sandwich (capture) approach because cytokines are soluble proteins present at low concentrations and require high specificity and a quantitative readout. In this setup, a capture antibody coated on the plate binds the cytokine from the serum or plasma. A second antibody, recognizing a different epitope on the same cytokine, is added and linked to an enzyme or detected with an enzyme-labeled secondary antibody. When the substrate for that enzyme is added, a color change occurs, and the intensity is proportional to the amount of cytokine present.

Crucially, a standard curve is run in parallel using known concentrations of the cytokine. By comparing the sample signal to this curve, you obtain an exact concentration value, typically expressed in pg/mL or ng/mL. The use of two antibodies increases specificity and helps distinguish the target cytokine from related proteins, while the standard curve provides the quantitative basis for measurement.

Other options don’t fit as well for this purpose. Western blot is more about identifying proteins and giving semi-quantitative results, not convenient, high-throughput quantification in patient samples. Agglutination involves visible clumping for certain antibody-antigen interactions and is not a reliable method for precise cytokine quantification. Flow cytometry analyzes cells or beads and can measure cytokines in cells or in bead-based assays, but it’s not the classic ELISA method used to quantify soluble cytokines directly in serum or plasma.