In a sandwich ELISA: (1) capture antibody is coated on a well, (2) sample antigen binds, (3) a detection antibody conjugated to an enzyme (e.g. HRP, alkaline phosphatase) binds the antigen, (4) enzyme substrate is added, producing a coloured product measured spectrophotometrically. The absorbance is proportional to antigen concentration. ELISA is widely used for diagnostics (HIV, pregnancy tests), food safety testing, and drug monitoring.