What Are Ligand Assays?

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  • Written By: Eliza Kay
  • Edited By: A. Joseph
  • Last Modified Date: 05 October 2019
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Assays are a method used in medicine to determine the presence and amount of a molecule in a biological substance. Different types of assays exist, and they are denoted by their name. Ligand assays specifically refer to the group of assays that measure the product from the chemical interaction between the molecule being measured and a reactive substance or receptor placed on the assay itself.

In the human body, a ligand is a signal-triggering molecule that fits to specific receptor sites located on cell walls. When a ligand fits into its specific receptor site, it causes the receptor’s physical shape to change which then triggers a biological signal to be sent out. In ligand assays, it is this cellular response that is measured.

Ligand assays can be used to measure a wide variety of substances in the human body, such as cells, drugs and hormones. Multiple kinds of biological ligand assays have been created, because the human body is very complex. The mechanism by which the measurement is made can be used to classify assays into three main classes, including competitive, non-competitive and miniaturized microarray assays.


On a competitive assay, both a radioactively labeled compound of similar structure to the molecule under investigation and the receptor binding site are provided on the assay. When the human sample is introduced to be measured on the assay, it must compete with the radioactively labeled molecules already on the assay to bind to the provided receptors. The amount in which the radioactively labeled molecules out-compete the sample’s receptor binding can provide medical professionals with information on the presence and quantity of the molecules in the sample. Competitive assays were first used in the mid-1950s to measure the concentration of hormones in humans, mainly thyroid hormone and breast cancer hormone receptor function.

A second type of ligand assay is the non-competitive assay. It works in a manner that is similar to that of the competitive assay, except that the radioactively labeled molecules on the assay are available in excess and therefore already out-compete the sample’s molecules. The degree to which the sample’s molecules are able to bind to the assay’s receptors can estimate the amount of the molecule present in the human sample. This assay technology became highly utilized in the mid-1980s.

Competitive and non-competitive ligand essays allow the investigation of one molecule at a time. The third class of assays permits the simultaneous measurement of multiple molecules from a small biological sample. This is done through the use of a miniaturized chip loaded with multiple tiny areas of specific receptors. These are called miniaturized microarrays or microspots. This third class of assay types lends the ability to measure multiple biological complexes from small samples, and it has become more widely used.


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