An immunoassay measures the natural reponse of a living organism to a foreign agent. For example, the common cold is caused by a virus. A virus is a piece of RNA (ribonucleic acid) with a protein coat surrounding it. When this virus enters the body it triggers the immune system to produce antibodies that will attach themselves to the virus or "antigen".

Immunoassays have been in use since the the early 1960's when radioimmunoassays were used to quantify insulin in plasma samples. Since their introduction, immunoassays have been used to detect and quantify hundreds of types of molecules both native to living organisms, such as hormones, and foreign molecules, such as pharmaceuticals. The molecules detected by immunoassays vary widely in size, chemical and physical properties, and biological activity.