Competitive Binding

A competitive binding assay is an immunoassay reaction that is based upon the competition of labeled and unlabeled ligand for a limited number of binding sites on the binder. A fixed amount of labeled ligand (tracer) and a variable amount of unlabeled ligand are incubated with the binder. Following the law of mass action, the amount of labeled ligand that can bind to the binder is a function of the total concentration of labeled and unlabeled ligand. As the concentration of unlabeled ligand is increased, less labeled ligand can bind to the binder and the measured response is decreased. The standard curve of a competitive binding assay has a negative slope and is generally symmetrical at the midpoint.