Immunoassays

An immunoassay is a biochemical test that measures the concentration of a target molecule by exploiting the highly specific interaction between an antibody and its antigen. Immunoassays use antibodies as affinity reagents to capture or detect the target and rely on a detectable signal produced by a label or a label-free method. Common labels include enzymes, fluorescent molecules, chemiluminescent reagents, or radioactive isotopes. The assay typically employs a solid phase to separate bound from free components and a readout that correlates with the amount of target present, enabling quantitative or qualitative results.

Two major formats are competitive and sandwich immunoassays. In sandwich immunoassays, a capture antibody binds the target, and a labeled detector antibody provides the signal, yielding a signal proportional to antigen concentration. In competitive immunoassays, labelled antigen competes with the sample antigen for limited binding sites, and the signal inversely reflects the target amount. Lateral flow assays provide rapid, often qualitative or semi-quantitative results, frequently using colloidal gold. Enzyme-linked immunosorbent assays (ELISA) and chemiluminescent or electrochemiluminescent immunoassays are common laboratory implementations.

Applications span clinical diagnostics—such as hormone, drug, infectious disease, and tumor-marker testing—along with environmental monitoring and food safety. Performance depends on reagent quality, assay design, and proper validation. Common considerations include specificity and cross-reactivity, sensitivity and dynamic range, matrix effects, and potential interference. Laboratories rely on calibration curves, controls, and standardized procedures. Immunoassays offer high sensitivity and throughput, but results can be affected by heterogeneity of samples and require appropriate handling of reagents and instrumentation.