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ELISA evolved in the late 1960s from RIA (radioimmunoassay) with the observation that either the antibody or the analyte (antigen) could be adsorbed to a solid surface and still participate in specific high affinity binding. The adsorption process facilitated the separation of bound and free analyte, a situation that proved difficult to engineer for many analytes with RIA. Over the intervening years, the term ELISA has been applied to a wide variety of immunoassays, some of which do not employ enzymes and some of which do not require the separation of bound and free analyte. The distinguishing feature of all of these assays remains the use of antibodies to detect an analyte.
The term ELISA was first used by Engvall & Perlma in 1971
direct elisa
Protocol step by step
APPLICATIONDr Dennis E Bidwell and Alister Voller created the ELISA test to detect various kind of diseases, such
as malaria, Chagas disease, and Johne's disease.[11] ELISA tests also are used as in in vitro diagnostics in medical laboratories. The other uses of ELISA include:
detection of Mycobacterium antibodies in tuberculosis
detection of rotavirus in feces
detection of hepatitis B markers in serum
detection of enterotoxin of E. coli in feces
detection of HIV antibodies in blood samples
Because the ELISA can be performed to evaluate either the presence of antigen or the presence of antibody in
a sample, it is a useful tool for determining serum antibody concentrations (such as with the HIV test[9] or West
Nile virus). It has also found applications in the food industry in detecting potential food allergens, such
as milk, peanuts, walnuts, almonds, and eggs.[10] ELISA can also be used in toxicology as a rapid presumptive
screen for certain classes of drugs.
The ELISA was the first screening test widely used for HIV because of its high sensitivity. In an ELISA, a
person's serum is diluted 400 times and applied to a plate to which HIV antigens are attached. If antibodies to
HIV are present in the serum, they may bind to these HIV antigens. The plate is then washed to remove all
other components of the serum. A specially prepared "secondary antibody" — an antibody that binds to other
antibodies — is then applied to the plate, followed by another wash. This secondary antibody is chemically
linked in advance to an enzyme.
Thus, the plate will contain enzyme in proportion to the amount of secondary antibody bound to the plate. A
substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in color or fluorescence.
ELISA results are reported as a number; the most controversial aspect of this test is determining the "cutoff"
point between a positive and a negative result.
A cutoff point may be determined by comparing it with a known standard. If an ELISA test is used for drug
screening at the workplace, a cutoff concentration, 50 ng/ml, for example, is established, and a sample
containing the standard concentration of analyte will be prepared. Unknowns that generate a stronger signal
than the known sample are "positive." Those that generate weaker signal are "negative".
LIMITATAIONSOne limitation of the ELISA technique is that it provides information on the presence of an
analyte but no information on its biochemical properties, such as molecular weight or its spatial
distribution in a tissue. To obtain this information one needs to perform other types of assays.
For example, blotting assays combine separations based on physical properties of the analyte
with detection techniques Immunohistochemical assays performed on tissue and cells provide
information on the specific location of an analyte. (see Immunohistochemical Staining:
Principles and Practices).Both of these techniques can also provide some quantitation of the
analyte, but not as accurately as ELISA.