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