Protection of the mammary gland against mastitis-causing pathogens is mediated by many factors in the gland and blood circulation. The professional phagocytic cells of bovine udder, polymorphonuclear neutrophils (PMN) and macrophages, comprise the first line of defense against invading mastitis pathogens. Most researchers now accept that the PMN is a key factor in the cows’ defense against intramammary infections. The PMN are the only leukocytes in the milk compartment that are capable of producing large amounts of reactive oxygen species (ROS) to kill phagocytosed bacteria. In this review, the role of PMN function as an effective defense against intramammary pathogens in dairy cows and physiopathological influencing factors on blood and milk PMN functions are discussed. Apart from playing a crucial role in the first line of defense mechanism, the PMN can also, indirectly, interfere with the complex interactions of second line of defense against pathogens. To minimize mammary tissue damage caused by bacterial toxins and oxidative products released by PMN, elimination of invading bacteria should proceed quickly. This can provide balance among inflammation reactions, bactericidal activity and tissue damage. The good balance between host-pathogen interactions might be affected by the physiological (e.g., stage and number of lactation) and pathological (e.g., local-systemic effect of mastitis) status of dairy cows. Hormones, metabolites and acute phase proteins also influence PMN functions, thereby affecting the outcome of mastitis. This is especially the case around parturition. PMN function in healthy cows after parturition is highly heritable and has been related to the cow’s susceptibility to clinical mastitis. Despite advances in veterinary science, nutrition and molecular biology, mastitis is still a very big problem in high yielding dairy cows. The long-term and fundamental solution for mastitis affecting high yielding dairy cows is to strengthen cows’ immune systems by means of attainable physio-immunological approaches. This requires a comprehensive study on the immunophysiological alterations throughout lactation and during mastitis. This review focuses on some factors affecting PMN functions during the lactation cycle and mastitis in high yielding dairy cows.

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