Immunoassay is a technique used to detect and quantify the presence of a substance (such as a protein or drug) in a sample by utilizing the interaction between an antibody and an antigen. This interaction produces a measurable signal, such as fluorescence or color change, that is proportional to the concentration of the target substance in the sample. Immunoassays are widely used in research, clinical diagnostics, and pharmaceutical development due to their specificity, sensitivity, and versatility. There are various types of immunoassays, including enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA), and immunofluorescence assays.