The bovine mammary gland is remarkable in that it produces over three litres of milk daily as an exocrine secretion, which passes through the teat into the external environment. Despite this substantial volume of fluid flow and that milk is one of the most nutritional mediums in which microorganisms can thrive, for the most part, this process occurs without colonisation of pathogens in the interior of the gland. Nevertheless, mastitis, when it does occur, is a significant dairy management problem. The major factor restricting ingress of pathogens into the gland has been assumed by most researchers in the field to be the length and diameter of the teat canal and the tightness of the teat sphincter. However, it has become increasingly clear in recent years that epithelial tissues have considerable innate immune functionality that contributes significantly to host defence. The presence of innate immune components within the teat end tissues has been only very superficially described to date. The presence of antimicrobial activity in the protein fraction of the teat canal lining has been previously demonstrated, as well as the presence of immune cells within some teat-end tissues, however, a detailed investigation into the identity of these proteins and cell types has not been undertaken. The aims of this thesis are to firstly, characterise the protein component of the teat canal lining (Chapter 3) and secondly, identify key immune-related cells in healthy lactating dairy cows (Chapter 4).0 A third aim is to assess changes in the cell population and immune-related factors that occur within teat-end tissues during mammary involution (Chapter 5). The fourth and final aim is to begin to characterise the immediate localised inflammatory response within teat-end tissues to the presence of mastitis-causing pathogens (Chapter 6), a question that has never been previously addressed. The approach used to address these aims were to (i) apply proteomics methodologies to the characterisation of the proteins within the teat canal lining, and (ii) examination of teat-end tissues by fluorescence immunohistochemistry using antibodies directed against known immune cell markers. The results revealed that aside from the major keratin proteins, the teat canal lining contained several other proteins and protein families (e.g. the S100 and serpin families) that differed from that of the cornified layer of the surrounding teat skin (Chapter 3). Immunofluorescent analysis revealed that there were substantial differences in the density and distribution of immune cells between the teat-end tissues. The Fürstenberg’s rosette is especially rich in antigen-presenting cells (Chapter 4). Drying-off of the udder (mammary involution) was associated with an increase in some immune cell types in the teat sinus, and an increased abundance of some antimicrobial proteins in the teat canal lining, but relatively few and minor additional changes (Chapter 5). The introduction of pathogens into the teat canal resulted in similar changes in the abundance of teat canal lining proteins, but no obvious changes in localised immune cell abundance or distribution after 24 hours exposure (Chapter 6). The presence of specific antimicrobial proteins in the teat canal lining, and the presence of immune cells within the Fürstenberg’s rosette and teat sinus epithelia demonstrate that these tissues play an active role in host defence. Thus, the research presented in this thesis has laid the foundation for a more complete understanding of host-defence functionality in the teat. This has implications for devising novel approaches to reducing susceptibility to mastitis in dairy cows.