Sandwich ELISA is the most frequently used immunoassay format for the detection of high molecular-weight antigens (e.g. proteins and peptides) and horseradish peroxidase is the most frequently used enzyme label. An antibody pair of the capture antibody (immobilized) and detection antibody (in solution) forms a “sandwich“ with a target antigen in between. Both polyclonal and monoclonal antibodies are used. The main features of those immunoassays are high sensitivity and specificity. The use of an HRP-labeled detection antibody (horseradish peroxidase) results in a fast, simple and sensitive immunoassay.