Upper image has had Citric acid HIER. Then incubated with stABCpx. Disclosed using DAB. Lower image was not subjected to HIER but, was otherwise treated identically to upper image.

Point is, Citric acid HIER discloses endogenous biotin.

An Avidin: biotin blocking procedure after HIER  but, prior to primary Ab incubation is recommended. One can buy such kits. However, it is quite easy to make your own for a much reduced cost.