The current protocols for blocking background staining in immunohistochemistry are based on conflicting reports. Background staining is thought to occur as a result of either non-specific antibody (Ab) binding to endogenous Fc receptors (FcRs) or a combination of ionic and hydrophobic interactions. In this study, cell and tissue samples were processed according to routine protocols either with or without a blocking step (goat serum or BSA). Surprisingly, no Abs in samples processed without a blocking step showed any propensity for non-specific binding leading to background staining, implying that endogenous FcRs do not retain their ability to bind the Fc portion of Abs after standard fixation. Likewise, we did not find any non-specific Ab binding ascribable to either ionic or hydrophobic interactions. We determined that traditionally used protein blocking steps are unnecessary in the immunostaining of routinely fixed cell and tissue samples.

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