The Sandwich Enzyme-Linked ImmunoSorbent Assay (ELISA) is a sensitive and robust method which measures the antigen concentration in an unknown sample. The antigen of interest is quantified between two layers of antibodies: the capture and the detection antibody. These antibodies must bind to non-overlapping epitopes on the antigen. Either monoclonal or affinity-purified polyclonal antibodies can be used as capture and detection antibodies, depending on cost, the dynamic range and the sensitivity of the final assay.