immunoassay
Meaning of immunoassay: Borrowing the immune system's precision to pick one molecule out of blood and measure how much is there.
Definition of immunoassay
An immunoassay is a laboratory test that uses antibodies to find and measure a specific substance in a sample of blood, urine or tissue. The antibody is chosen because it binds only its intended target, so it can pick that one molecule out of a complex mixture. A label attached to the antibody produces a color, glow or signal in proportion to the amount present.
How does it work?
Antibodies are proteins the immune system builds to recognize one shape. A laboratory can produce large quantities of an antibody against almost any target and then use it as a molecular hook. In a common arrangement the antibody is fixed to the wall of a well, the sample is added, the target sticks, everything else is washed away, and a second labeled antibody is added that grips a different part of the same target. The label creates a color, a flash of light or an electrical signal, and its strength is compared against standards of known concentration.
What are the main formats?
| Format | How it reads out | Typical setting |
|---|---|---|
| ELISA | An enzyme produces a color change | Hospital and research laboratories |
| Chemiluminescent assay | The label emits light | Automated high volume analyzers |
| Lateral flow | A colored line appears on a strip | Home pregnancy and rapid infection tests |
| Radioimmunoassay | A radioactive label is counted | Historic, now largely replaced |
A home pregnancy test and a hospital hormone analyzer rest on the same principle and differ mainly in how the signal is read and how precisely it is measured.
What is measured this way?
Hormones, drug levels, the cardiac proteins checked after a suspected heart attack, infection and inflammation markers, allergy antibodies and proteins released by tumors are all measured by immunoassay. The same chemistry is applied to tissue on a slide, where a stain reveals whether cells carry a particular protein. That is how estrogen receptors are detected in breast cancer, a result that decides whether hormone blocking treatment is likely to help.
What makes results go wrong?
The method depends entirely on the antibody binding only what it should. A closely related molecule that binds as well causes cross reaction and a falsely raised result. Some people carry antibodies that bridge the test antibodies directly, generating a signal with no target present at all. At very high concentrations the target can saturate both antibodies separately and stop the sandwich forming, so an extreme result reads as low. A result that does not fit the clinical picture is therefore repeated, often by a different method.
Used in a sentence
The hormone level was measured by immunoassay and repeated after dilution to exclude interference.
Good to know
An immunoassay result rests on the antibody binding only its intended target, so cross reacting substances and interfering antibodies can produce results that do not match the clinical picture and are confirmed by another method.
Word origin
Where “immunoassay” comes from
Immuno- comes from the Latin immunis, meaning exempt, and assay from the Latin exagium, meaning a weighing.