Two types of immunoassays. (A) Competitive immunoassay. The sample is incubated with an anti-cortisol antibody (Ab) (for example), and a tracer (cortisol labeled with a measurable signal: enzyme, fluorescent, or luminescent compound isotope) is added. Competition occurs between the cortisol in the sample and the labeled cortisol for the Ab-binding sites. These cortisol molecules bound to the Ab are captured on a solid phase. Unlike sandwich assays, the higher the cortisol concentration in the sample, the lower the signal-labeled cortisol bound to the antibody and finally linked to the solid phase. (B) Sandwich immunoassay. TSH (for example) is “sandwiched” between two different antibodies: one is labeled with a signal to be measured (luminescent or fluorescent compound, enzyme, isotope) and the other one, named the “capture antibody,” will allow the separation of the immune complexes on a solid phase. The higher the TSH concentration, the higher the signal linked to the solid phase will be.
Abbreviations: Ab, antibody; Ag, antigen; TSH, thyroid-stimulating hormone.
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