enzyme linked immunosorbent assay elisa PSI-MI MI:0411 Following non-covalent binding of a purified primary ligand to a solid phase, a blocking reagent is added to prevent any non-specific binding. A specific antigen is then allowed to bind to the primary ligand. Unbound antigen is removed by washing and a secondary antibody conjugated to an enzyme (e.g. horseradish peroxidase) is added. Following a washing step to remove unbound secondary ligand, the extent to which a chromogenic substrate (e.g. 3,3', 5,5' tetramethyl benzidine chromogen [TMB]) is converted to a soluble coloured product by the conjugated enzyme in a given time is determined by spectrophotometry using a standard microplate absorbance reader. A similar type of approach can be utilized to detect enzymatic activities. The substrate, attached to a solid phase is incubated in the presence of the enzyme and the enzymatic modification is monitored by an antibody that is specific for the modified substrate (for instance a phosphorylated protein). ELISA identification by antibody solid phase assay