Enzyme immunoassay (EIA) is a laboratory technique used to detect and quantify specific proteins in biological samples, such as blood or urine. This method is based on the principle of antigen-antibody binding, where an enzyme-linked antibody is used to detect the presence of the target protein. In an EIA, the target protein (antigen) is bound to a solid surface, such as a microplate, and then a specific enzyme-linked antibody is added to the sample. If the target protein is present in the sample, it will bind to the antibody-enzyme complex, resulting in the formation of a colored or fluorescent product that can be measured using a spectrophotometer. EIA is commonly used in medical diagnostics, drug testing, and research to detect and quantify various proteins, hormones, and antibodies in biological samples. It is a sensitive and specific technique that is widely used in clinical laboratories and research settings.