Labelled Antibodies and Immunological Assay Systems

Abstract

ALL assay procedures which do not rely on the direct detection of a specific property of the unknown substance must depend on the reaction of an unknown with a reagent to give a product which can then be assayed. The product can be estimated directly or indirectly (for example, as a result of a further reaction). If the unknown is regenerated after the primary reaction it may then react with more reagent and a cycling assay is established. The advantage of such a system is that one molecule of the unknown gives rise to several molecules of product with a consequent increase in sensitivity. The reagent can be as diverse as a dye, neutron irradiation, an enzyme substrate or an antibody. Adequate assay systems must be shown to have suitable specificity, sensitivity, precision, range and convenience. In order to obtain maximum sensitivity and precision: (1) all the unknown should be reacted at least once and preferably several times; (2) the amount of product should be assayed by a procedure which gives a low background and shows changes in direct proportion to the change in product concentration; (3) the property measured should be capable of detection at very low concentrations of the product. The use of one or more cycling reactions is one well recognized way of achieving suitable amplification (compare the assay of metabolic intermediates and enzymes).