Discussion
Which of the following technique is used in assays for small molecules where only a single epitope is present on the antigen ?
Answer: C
The technique is competitive ELISA.
A sandwich ELISA captures the antigen between two antibodies, which needs at least two separate epitopes on the antigen. A small molecule (a drug, a hormone or a toxin) is too small to bind two antibodies at once and often has only one epitope.
In a competitive ELISA, the antigen in the sample competes with a labelled antigen for a limited number of antibody sites, so only one binding site is needed; a stronger sample means a weaker signal. That is why drug-screening immunoassays are competitive.
The double and triple antibody sandwich formats both need two epitopes, and DELFIA is a lanthanide-label detection system usually run as a sandwich, not the format defined by single-epitope antigens.
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