Immunohistochemistry (IHC) experiments often contain one or more blocking steps to reduce background signal and false positives. Blocking with sera or a protein blocking agent is essential to prevent non-specific binding of antibodies to tissue or to Fc receptors. Serum is a common blocking agent as it contains antibodies that bind to non-specific sites. The Blocking sera should be from the same species from which the secondary antibody was raised.
Proteins such as Bovine Serum Albumin (BSA) may also be used to block non-specific antibody binding. This is simpler to use as there is often no need to match the reagent to the species of the secondary antibody.