A radiobinding assay is a method of detecting and quantifying antibodies targeted towards a specific antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

. As such, it can be seen as the inverse of radioimmunoassay

Radioimmunoassay

Radioimmunoassay is a very sensitive in vitro assay technique used to measure concentrations of antigens by use of antibodies...

, which quantifies an antigen by use of corresponding antibodies.

Technique

The corresponding antigen is radiolabeled and mixed with the fluid that may contain the antibody, such as blood serum

Blood serum

In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...

 from a person. Presence of antibodies cause precipitation of antibody-antigen complexes that can be collected by centrifugation

Centrifugation

Centrifugation is a process that involves the use of the centrifugal force for the sedimentation of mixtures with a centrifuge, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture...

 into pellets. The amount of antibody is proportional to the radioactivity of the pellet, as determined by gamma counting.