Yesterday I neglected to add in my answer to IHC background and hyperimmune
serum that you CANNOT add 2% normal serum to the biotinylated linking
antibody when hyperimmune sera are employed.  You can do this only when
using antibodies not made in the case species.  If you add normal serum to
the link Ab for hyperimmune serum, you will bind up all the linking
antibody.

For instance, using hyperimmune serum for a pig virus...  hyperimmune serum
is applied to the pig tissue; then biotinylated goat anti-pig IgG is
applied; then, streptavidin HRP.  If you put 2% normal pig serum in with the
biotinylated goat anti-pig IgG, you'll bind up the goat linking antibodies,
and get no staining results.

Jan Shivers
Vet Diagnostic Lab
Univ. of Minnesota
shive003@tc.umn.edu