Dictionary Definition
immunoassay n : identification of a substance
(especially a protein) by its action as an antigen; "PSA in the
blood can be measured with an immunochemical assay" [syn: immunochemical
assay]
User Contributed Dictionary
Noun
- A test for the presence of a substance using the reaction of an antibody to its antigen, making use of the high selectivity of components of biological immune systems.
Extensive Definition
An immunoassay is a biochemical test that
measures the concentration of a
substance in a biological liquid, typically serum or
urine, using the reaction
of an antibody or
antibodies to its antigen. The assay takes advantage of the
specific binding of an antibody to its antigen. Monoclonal
antibodies are often used as they only usually bind to one site
of a particular molecule, and therefore provide a more specific and
accurate test, which is less easily confused by the presence of
other molecules. The antibodies picked must have a
high affinity for the antigen (if there is antigen available, a very high
proportion of it must bind to the antibody).
Both the presence of antigen or antibodies can be measured.
For instance, when detecting infection the presence of antibody against the pathogen is measured. For
measuring hormones such
as insulin, the insulin acts as the antigen.
For numerical results, the response of the fluid
being measured must be compared to standards of a known
concentration. This is usually done though the plotting of a
standard curve on a graph, the position of the curve at response of
the unknown is then examined, and so the quantity of the unknown
found.
Detecting the quantity of antibody or antigen can be achieved by a
variety of methods. One of the most common is to label either the
antigen or antibody. The label may consist
of an enzyme (see
enzyme
immunoassay (EIA)), radioisotopes such as
I-125 Radioimmunoassay
(RIA), magnetic labels (magnetic immunoassay - MIA) or fluorescence. Other
techniques include agglutination,
nephelometry,
turbidimetry and
Western
Blot.
Types
Immunoassays can be divided into those that involve labelled reagents and those which involve non-labelled reagents. Those which involve labelled reagents are divided into homogenous and heterogeneous (which require an extra step to remove unbound antibody or antigen from the site, usually using a solid phase reagent) immunoassays. Heterogeneous immunoassays can be competitive or non-competitive.- In a competitive immunoassay, the antigen in the unknown sample competes with labeled antigen to bind with antibodies. The amount of labeled antigen bound to the antibody site is then measured. In this method, the response will be inversely proportional to the concentration of antigen in the unknown. This is because the greater the response, the less antigen in the unknown was available to compete with the labeled antigen.
- In noncompetitive immunoassays, also referred to as the "sandwich assay," antigen in the unknown is bound to the antibody site, then labeled antibody is bound to the antigen. The amount of labeled antibody on the site is then measured. Unlike the competitive method, the results of the noncompetitive method will be directly proportional to the concentration of the antigen. This is because labeled antibody will not bind if the antigen is not present in the unknown sample.
Because homogeneous assays do not require this step,
they are typically faster and easier to perform.
SHUMI is the most commonly used form of
immunoassays.
See also
External links
immunoassay in Czech: Imunoassay
immunoassay in German: Immunoassay
immunoassay in Indonesian: Imunoasai
immunoassay in Japanese: 免疫学的検定
immunoassay in Swedish:
Immunoassay