An ELISA substrate is a reagent used to produce a detectable signal in an ELISA (Enzyme-Linked Immunosorbent Assay). When an enzyme, such as horseradish peroxidase (HRP), conjugated to an antibody or antigen, reacts with the substrate, it triggers a color change, fluorescence, or chemiluminescence. This reaction allows for the quantitative measurement of the target molecule in the sample. The choice of substrate depends on the type of detection method being used.