Radioimmunoassay: Definition & Method

Instructor: Lauren Posey

Lauren has taught intermediate reading in an English Language Institute, and she has her Master's degree in Linguistics.

How can scientists measure substances in a person's blood? In this lesson you'll learn about radioimmunoassays, a commonly used method of detecting substances in blood, especially antigens.

Investigating Immune Responses

You are probably familiar with the basic function of your immune system, such as how it detects foreign and potentially harmful substances and removes them from the bloodstream. Substances that cause the body to have an immune response are called antigens. Antigens activate your body's white blood cells, which then produce antibodies, or proteins that find and attach to specific antigens in order to get rid of them.

Now, say you want to measure the amount of a specific antigen that someone has in their blood. How would you do it? One common and highly effective method is a radioimmunoassay. In this method, radioactive versions of a specific antigen are used to determine how much of the original antigen a person has in their bloodstream.

How It Works

Radioimmunoassays rely on the fact that a specific type of antibody will only bind to a specific type of antigen, and vice versa.

Antibodies are highly specific.
Antigen binding

A blood sample is taken from the patient and the relevant antibodies are added. The type of antibody is specific to the type of antigen being measured. The sample is then incubated for a while to give the antigens time to bind to the antibodies. The more antigens in the blood sample, the fewer free antibodies there will be.

Once the original antigens have had a chance to bind to the antibodies, scientists create a radiolabeled version of the antigen. When something is radiolabeled, it is marked with a radioactive 'label,' making it easy to find and follow in a blood sample. The marked antigens are then added to the incubated sample. The radioactive antigens will bind to any free antibodies left. A higher concentration of unmarked antigens means there will be more radioactive antigens left unbound.

The bound antibodies and antigens are heavier than the remaining free radioactive antigens, so the bound antibodies are separated out using a centrifuge. Scientists can then measure how radioactive the collection of bound antibodies is. Higher radioactivity means there are more bound radioactive antigens. This in turn means there were fewer unmarked antigens in the original sample.

Essentially, radioimmunoassays use measures of radioactivity to calculate the amount of a specific antigen in a blood sample.

Advancing Science

So, what is the point? Why would you want to know the level of a specific antigen in someone's blood? Actually, radioimmunoassays have a number of incredibly important applications. For one, the inventors of the radioimmunoassay process (Yalow and Berson) used it to detect insulin levels. This helped them show that adult onset diabetes is actually caused by the body not properly using the insulin it produces. Prior to radioimmunoassays, it was thought that diabetes was caused solely by a lack of insulin.

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