Using immunoassays, molecules of biological interest can be quantified based on the produced antibody reagents' specificity and selectivity. The structure of antibodies and other immunoassays, such as ELISA, RIA, and bioluminescence assay, are detailed along with the preparation process, types of assays if available, basic uses, and even drawbacks are briefly discussed. Each type of ELISA, including sandwich, competitive, and indirect ELSIA, is briefly described along with how it is prepared.