Localization of FcRn in bovine, sheep and water buffalo mammary tissue indicates that the receptor is homogeneously distributed throughout the epithelial cells prior to parturition, but primarily localized at the apical surface of the mammary epithelial cells after parturition [136] [137] [138] [139] . While this type of observation corroborates the conclusion that FcRn plays an important role in IgG transport during colostrum formation, at least in ruminant species, the precise meaning of this redistribution of FcRn staining in mammary cells remains to be determined. It is also interesting to note that the initial report of this distribution pattern in sheep mammary epithelium included the observation that the staining pattern became diffuse within the cells during mammary involution [136, 137] . Transport of IgG also may increase transiently in mammary secretions during involution in cattle [140] . Hormonal and local factors have been implicated in the control of immunoglobulin transport during colostrum formation [32] .