What are the types of immunoassay?

The five most-widely used immunoassays include: 

1. Enzyme-Linked Immunosorbent Assay (ELISA): A highly versatile and widely-used immunoassay, ELISA involves immobilizing antigens or capture antibodies on the surface of microplates. On incubating the immobilized antigens with target antigen-specific antibodies, the resultant antigen-antibody can be visualized using various detecting techniques such as colorimetric, fluorescent, chemiluminescent, and chromogenic assays. Because of its high sensitivity and high specificity, ELISA is the preferred immunoassay in a wide range of biomedical applications including the high-throughput screening of antibodies and drugs.  
2. Radioimmunoassay (RIA): Radioimmunoassay uses radioactive isotopes or radioisotopes as labels to detect specific antigens. This technique is based on the competition between a radiolabeled antigen and an unlabeled antigen for binding to a specific antibody. A high throughput technique, RIA is used in laboratories to quantify hormones, viral antigens, and drugs. 
3. Fluorescence Immunoassay: This type of immunoassay uses fluorescent labels to detect a range of antigens. Measuring the fluorescence intensity when the fluorescent label binds to the antigen enables researchers to quantify the analyte of interest in the sample. Fluorescence immunoassay offers high sensitivity and is often used in medical diagnostics, environmental testing, and molecular biology research
4. Chemiluminescence Immunoassay (CLIA): CLIA utilizes light-emitting reactions to detect antigens, hormones, and peptides in a range of samples. In chemiluminescence immunoassays, a substrate that is converted into a reaction product by enzymes, emits light instead of producing a specific color. CLIA is known for its high sensitivity and is employed in clinical laboratories for diagnostic tests, including hormone assays and infectious disease detection.
5. Bead-Based immunoassay: In this type of immunoassay, particle beads are covered with antibodies specific to the target antigen. The coated beads are then incubated with the sample, which results in the formation of an antigen-antibody complex. Beads are differentiated based on color, bead size, fluorescence, and luminescence. The unbound beads are counted using a cell counter, giving the concentration of the antigen of interest. 

Immunoassays play a pivotal role in accurate diagnoses, understanding how drugs work, and monitoring levels of therapeutic drugs in the body, all of which help scientists and doctors better understand and treat various illnesses, ensuring more effective and safer medications. These bioanalytical tests are used in a wide range of medical areas from cancer diagnostics and pregnancy testing to monitoring the effectiveness of drug therapies.