The enzyme is attached to the labelled antibody. DAB and Fast red are substrates for the colour change.

In amplification IHC. Biotin is attached to the labelled antibody and then an enzyme labelled avidin or streptavidin is added to form a chain of labelled antibodies. This amplifies the signal

From: https://www.mblbio.com/bio/g/support/method/immunohistochemistry.html From: https://www.bosterbio.com/protocol-and-troubleshooting/immunohistochemistry-ihc-principle?srsltid=AfmBOopxyXQiCuZQFIbKxBjxMgaPfl5KhJzJDn_QCbtTlkgJ9_U0MefL