Some tissue sections contain endogenous biotin, biotin binding proteins, lections or non-specific binding substances. Tissues may also bind streptavidin, biotin, peroxidase biotin, peroxidase streptavidin in streptavidin biotin based Immunohistochemistry. These tissues will give high in the absence of biotinylated secondary antibodies. It may be necessary to block the tissue with streptavidin, followed by biotin. These reagents are added before the primary antibody.