Immunassays

Immunassays are biochemical tests that use antigen–antibody interactions to detect or quantify substances in a sample. They rely on a specific antibody to recognize a target analyte and on a detectable signal produced by a label or by binding events. Immunoassays can be designed to detect either the target antigen or specific antibodies against it, making them versatile for diagnostic and research applications.

Formats fall broadly into competitive and non-competitive (sandwich) assays, and into heterogeneous (solid-phase separated) or homogeneous (no separation) formats. In sandwich assays, a capture antibody binds the analyte and a labeled detection antibody provides the signal, making the method well suited for larger targets. In competitive assays, labeled antigen competes with the sample antigen for binding sites, and the signal is inversely related to the analyte concentration.

Common methods include enzyme-linked immunosorbent assays (ELISA), immunoradiometric or radioimmunoassays (RIA/IRMA), fluorescence immunoassays (FIA), and chemiluminescent immunoassays, as well as lateral flow immunoassays (rapid tests). ELISA typically uses an enzyme-substrate reaction to produce a measurable color change, while lateral flow assays yield a visual line for rapid, qualitative or semi-quantitative results.

Key performance characteristics include sensitivity, specificity, dynamic range, detection limit, and precision. Assays rely on calibration with standards and appropriate controls; potential interferences include cross-reactivity, matrix effects, and certain antibodies that cause false results. Safety concerns arise with radioactive labels in older formats.

Applications span clinical diagnostics (pregnancy tests, infectious disease serology, autoimmune panels, therapeutic drug monitoring), food safety, environmental testing, and basic research. Immunoassays offer high specificity and adaptability to automation and high-throughput workflows, but require robust controls and careful interpretation of results.