What is HRP conjugated secondary antibody?

HRP conjugated secondary antibody is a secondary antibody that has been tagged or conjugated with Horseradish Peroxidase (HRP).

HRP is an enzyme that occurs naturally in horseradish. Secondary antibodies are conjugated with HRP to increase their sensitivity and signal amplification when used in a number of assays including ELISA, Western Blotting, and microscopy. 

In all assays, the HRP conjugated secondary antibody does not bind directly to the target antigen. Instead it binds to a primary antibody that is directly bound to the target antigen. The improved sensitivity and signal amplification is because of multiple secondary antibody conjugates binding to one single primary antibody, bringing a more diverse range of reporter molecules to the antibody-antigen complex.