Radioimmunoassay (RIA) is a very sensitive technique used to measure concentrations of antigens (for example, hormone levels in the blood) without the need to use a bioassay.

Although the RIA technique is extremely sensitive and extremely specific, it requires specialized equipment and is costly. It also requires special precautions, since radioactive substances are used. Therefore, today it has been largely supplanted by the ELISA method, where the antigen-antibody reaction is measured using colorimetric signals instead of a radioactive signal.

The RAST test (radioallergosorbent test) is an example of radioimmunoassay. It is used to detect the causative allergen for an allergy.

Contents 1 Method 2 History 3 References 4 External links

[edit] Method

To perform a radioimmunoassay, a known quantity of an antigen is made radioactive, frequently by labeling it with gamma-radioactive isotopes of iodine attached to tyrosine. This radiolabeled antigen is then mixed with a known amount of antibody for that antigen, and as a result, the two chemically 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 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 from the unbound ones, and the radioactivity of the free antigen remaining in the supernatant is measured. Using known standards, a binding curve can then be generated which allows the amount of antigen in the patient's serum to be derived.

[edit] History

It was developed by Rosalyn Yalow and Solomon Aaron Berson in the 1950s.^[1] In 1977, Rosalyn Sussman Yalow received the Nobel Prize in Medicine for the development of the RIA for insulin: the precise measurement of minute amounts of such a hormone was considered a breakthrough in endocrinology.

With this technique, separating bound from unbound antigen is crucial. Initially, the method of separation employed was the use of a second "anti-antibody" directed against the first for precipitation and centrifugation. The use of charcoal suspension for precipitation was extended but replaced later by Drs. Werner and Acebedo at Columbia University for RIA of T3 and T4.^[2] An ultramicro RIA for human TSH was published in BBRC (1975) by Drs. Acebedo, Hayek et al.^[3]

[edit] References

1. ^ Yalow RS, Berson SA. Immunoassay of endogenous plasma insulin in man. J Clin Invest 1960;39:1157-75. PMID 13846364.
2. ^ Werner SC, Acebedo G, Radichevich I (1974). "Rapid radioimmunoassay for both T4 and T3 in the same sample of human serum". J. Clin. Endocrinol. Metab. 38 (3): 493–5. PMID 4815178. 
3. ^ Acebedo G, Hayek A, Klegerman M, Crolla L, Bermes E, Brooks M (1975). "A rapid ultramicro radioimmunoassay for human thyrotropin". Biochem. Biophys. Res. Commun. 65 (2): 449–56. doi:10.1016/S0006-291X(75)80168-9. PMID 1148002. 

[edit] External links

• MeSH Radioimmunoassay
• Radioimmunoassay- procedure
• http://www.lib.mcg.edu/edu/esimmuno/ch4/radassay.htm

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