(df) Immunostaining of routinely formaldehyde-fixed paraffin-embedded human tissue sections. of non-specific binding of Abs, the attraction of primary and secondary Abs to endogenous Fc receptors (FcRs) is usually thought to be the main source of SEDC unwanted staining. FcRs are structures on CPI 4203 the surface of certain cells that bind the Fc region of Abs. Cross linking of Ab bound by FcRs provides an important link between the cellular and humoral branches of the immune system by inducing several responses, including phagocytosis, endocytosis, antibody-dependent cytotoxicity, release of inflammatory mediators, and enhancement of antigen presentation1. The nature of the response depends primarily around the cell type on which these FcRs are expressed. There are several types of FcR, which are classified on the basis of the type of immunoglobulins that they recognise2. FcRs for immunoglobulin G (IgG), the most common class of Ab used in immunohistochemistry, are designated Fc-gamma receptors (FcRs). Other FcRs are expressed on multiple cell types and are similar in structure to MHC class I. Being involved in antigen presentation, these receptors can also bind IgG3. It is theorised that FcRs bind the Fc region of Abs not onlyin vivobut also during immunohistochemical assays of cell and tissue samples. This concept has been pointed out in all publications regarding immunohistochemistry since its inception half a century ago4,5,6,7, but we have been unable to find the original source of the idea. It is thought that preincubation of a histological sample with 510% normal serum from the species that this secondary Ab is derived from will prevent non-specific binding of secondary Abs to CPI 4203 endogenous FcRs. This makes little sense for the immunohistochemical staining of human cell and tissue samples, as the vast majority of secondary Abs used in human immunohistopathology are derived from goats, and goat CPI 4203 serum has long been reported not to bind to FcRs on human cells8. Preincubation with solutions made up of normal goat serum have also been assumed to prevent background staining that might result from ionic and hydrophobic interactions5. Blocking the non-specific background due to FcRs or ionic and hydrophobic interactions is considered an obligatory step prior to incubation with primary Ab. This can be observed in immunohistochemical protocols in all contemporary Ab manufacturers’ catalogues (e.g., Dianova, ZytoMed and Jackson ImmunoResearch Laboratories Inc.), as well as on the popular IHC WORLD homepage CPI 4203 and the homepages of the Ab manufacturers. All Ab manufacturers offer their own ready-to-use blocking solutions, and their formulations are trade secrets in many cases. In spite of the fact that goat serum does not bind to FcRs on human cells8, goat serum remains the most popular blocking agent in human immunohistopathology. Some histochemists prefer FcR blocking with normal swine or rabbit serum9, but do not provide any experimental support for their preference. Additionally, more complicated blocking strategies have been reported, such as employing papain-digested fragments of unlabeled secondary Ab enriched with Fc fragments of the same IgG10. In theory, the most rational approach to prevent CPI 4203 the possible nonspecific background due to FcR binding would be the use of F(ab)2fragments of Ab instead of the whole IgG molecule11, provided that the endogenous FcRs do retain their ability to.