ELISA stands for enzyme-linked immunosorbent assay. It is a workhorse of medical diagnostics. It detects proteins, antibodies, and other molecules using antibodies linked to enzymes. When the target is present, the enzyme reacts with a substrate and produces a color change. The intensity of the color indicates how much target is present. It is simple, sensitive, and cheap.

The assay comes in several formats. In a direct ELISA, the antigen is immobilized on a plate and detected by an enzyme-linked antibody. In an indirect ELISA, a primary antibody binds the antigen, and a secondary enzyme-linked antibody binds the primary. In a sandwich ELISA, the antigen is captured between two antibodies, which improves specificity. In a competitive ELISA, the target competes with a labeled version for binding sites.

ELISA is used to diagnose HIV, hepatitis, Lyme disease, and many other infections. It is used to measure hormones, cytokines, and tumor markers. It is used in food safety to detect allergens and contaminants. It is used in research to quantify proteins in samples. It is one of the most widely used assays in the world.

The technique is not perfect. It can produce false positives and false negatives. Cross-reactivity can occur. It requires careful optimization and validation. But it remains a cornerstone of diagnostics because it works.