A radioimmunoassay (RIA) is an

immunoassay An immunoassay (IA) is a biochemical test that measures the presence or concentration of a macromolecule or a small molecule in a solution through the use of an antibody (usually) or an antigen (sometimes). The molecule detected by the immunoassay ...

that uses radiolabeled molecules in a stepwise formation of

immune complex An immune complex, sometimes called an antigen-antibody complex or antigen-bound antibody, is a molecule formed from the binding of multiple antigens to antibodies. The bound antigen and antibody act as a unitary object, effectively an antigen of ...

es. A RIA is a very sensitive

in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...

assay technique used to measure concentrations of substances, usually measuring

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

concentrations (for example,

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

levels in

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...

) by use of

antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

. The RIA technique is extremely sensitive and extremely

specific Specific may refer to: * Specificity (disambiguation) * Specific, a cure or therapy for a specific illness Law * Specific deterrence, focussed on an individual * Specific finding, intermediate verdict used by a jury in determining the final ...

, and although it requires specialized equipment, it remains among the least expensive methods to perform such measurements. It requires special precautions and licensing, since radioactive substances are used. In contrast, an immunoradiometric assay (IRMA) is an immunoassay that uses radiolabeled molecules but in an immediate rather than stepwise way. A radioallergosorbent test (RAST) is an example of radioimmunoassay. It is used to detect the causative

allergen An allergen is an otherwise harmless substance that triggers an allergic reaction in sensitive individuals by stimulating an immune response. In technical terms, an allergen is an antigen that is capable of stimulating a type-I hypersensitivi ...

for an

allergy Allergies, also known as allergic diseases, are various conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include Allergic rhinitis, hay fever, Food allergy, food al ...

Method

Classically, to perform a radioimmunoassay, a known quantity of an

is made

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

, frequently by labeling it with gamma-radioactive

isotopes of iodine There are 40 known isotopes of iodine (53I) from 108I to 147I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, 129I, has a half-life of 16.14 million y ...

, such as 125-I, or

tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...

attached to

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...

. This radiolabeled antigen is then mixed with a known amount of

antibody An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as pathogenic bacteria, bacteria and viruses, includin ...

for that antigen, and as a result, the two specifically bind to one another. Then, a sample of serum from a patient containing an unknown quantity of that same antigen is added. This causes the unlabeled (or "cold") antigen from the serum to compete with the radiolabeled antigen ("hot") for antibody binding sites. As the

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

of "cold" antigen is increased, more of it binds to the antibody, displacing the radiolabeled variant, and reducing the ratio of antibody-bound radiolabeled antigen to free radiolabeled antigen. The bound antigens are then separated and the radioactivity of the free(unbound) antigen remaining in the

supernatant In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

is measured using a gamma counter. This value is then compared to a standardised calibration curve to work out the concentration of the unlabelled antigen in the patient serum sample. RIAs can detect a few picograms of analyte in an experimental tubes if using antibodies of high affinity. This method can be used for any biological molecule in principle and is not restricted to serum antigens, nor is it required to use the indirect method of measuring the free antigen instead of directly measuring the captured antigen. For example, if it is undesirable or not possible to radiolabel the antigen or target molecule of interest, a RIA can be done if two different antibodies that recognize the target are available and the target is large enough (e.g., a protein) to present multiple

epitope An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that binds to the epitope is called a paratope. Although e ...

s to the antibodies. One antibody would be radiolabeled as above while the other would remain unmodified. The RIA would begin with the "cold" unlabeled antibody being allowed to interact and bind to the target molecule in solution. Preferably, this unlabeled antibody is immobilized in some way, such as coupled to an

agarose Agarose is a heteropolysaccharide, generally extracted from certain red algae. It is a linear polymer made up of the repeating unit of agarobiose, which is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agarose is on ...

bead, coated to a surface, etc. Next, the "hot" radiolabeled antibody is allowed to interact with the first antibody-target molecule complex. After extensive washing, the direct amount of radioactive antibody bound is measured and the amount of target molecule quantified by comparing it to a reference amount assayed at the same time. This method is similar in principle to the non-radioactive sandwich

ELISA The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay is a solid-phase type of enzyme immunoassay (EIA) to detect the presence of ...

method.

History

This method was developed by

Solomon Berson Solomon Aaron Berson (April 22, 1918 – April 11, 1972) was an American physician and scientist whose discoveries, mostly together with Rosalyn Yalow, caused major advances in clinical biochemistry.Rall JE. ''Solomon A. Berson''. In "Biographic ...

and

Rosalyn Sussman Yalow Rosalyn Sussman Yalow (July 19, 1921 – May 30, 2011) was an American medical physicist, and a co-winner of the 1977 Nobel Prize in Physiology or Medicine (together with Roger Guillemin and Andrew Schally) for development of the radioimmunoass ...

at the Veterans Administration Hospital in

the Bronx The Bronx ( ) is the northernmost of the five Boroughs of New York City, boroughs of New York City, coextensive with Bronx County, in the U.S. state of New York (state), New York. It shares a land border with Westchester County, New York, West ...

, New York. This revolutionary development earned Dr. Yalow the Nobel Prize for Medicine in 1977, the second woman ever to win it. In her acceptance speech, Dr. Yalow said, "The world cannot afford the loss of the talents of half its people if we are to solve the many problems which beset us." Yalow shared the Nobel Prize with

Roger Guillemin Roger Charles Louis Guillemin (; January 11, 1924 – February 21, 2024) was a French-American neuroscientist. He received the National Medal of Science in 1976, and the Nobel Prize in Physiology or Medicine, Nobel Prize for Medicine in 1977 for ...

, and Andrew Schally who earned the prize based on their research into "the peptide hormone production of the brain".

References

steps i
radioimmunoassay
technique

External links

*

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