An immunoassay is a chemical test used to detect the exact concentration of a single substance in a mixture of complex substances.  It is frequently used to test blood serum and urine.  It takes advantage of the fact that antibodies will bind to one, and only one, disease causing element or "antigen".  A such, the technican can use the antibody to find the antigen, or the antigen to find the antibody in a particular sample.  

Historically, the concentration of the analyzed substance was determined by examining how light travelled through the treated sample.  However, in modern use, many substances are used that can be easily tracked in the sample, such as radioactive isotopes, dyes, magenetic particles and heavy metal particles.  

Most often, immunoassays come in kits that contain several dozen different testing elements, each of which tests for a different substance, except for a few which act as a control.  

Immunoassay at Wikipedia

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