Role of neutrophils in protection of udder from infection in high yielding dairy cows

Protection of the mammary gland against mastitis-causing pathogens is mediated by many factors in thegland and blood circulation. The professional phagocytic cells of bovine udder, polymorphonuclearneutrophils (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 intramammaryinfections. The PMN are the only leukocytes in the milk compartment that are capable of producing largeamounts of reactive oxygen species (ROS) to kill phagocytosed bacteria. In this review, the role of PMNfunction as an effective defense against intramammary pathogens in dairy cows and physiopathologicalinfluencing factors on blood and milk PMN functions are discussed. Apart from playing a crucial role in thefirst line of defense mechanism, the PMN can also, indirectly, interfere with the complex interactions ofsecond line of defense against pathogens. To minimize mammary tissue damage caused by bacterial toxinsand oxidative products released by PMN, elimination of invading bacteria should proceed quickly. This canprovide balance among inflammation reactions, bactericidal activity and tissue damage. The good balancebetween host-pathogen interactions might be affected by the physiological (e.g., stage and number oflactation) 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 ofmastitis. This is especially the case around parturition. PMN function in healthy cows after parturition ishighly heritable and has been related to the cow’s susceptibility to clinical mastitis. Despite advances inveterinary science, nutrition and molecular biology, mastitis is still a very big problem in high yielding dairycows. The long-term and fundamental solution for mastitis affecting high yielding dairy cows is to strengthencows’ immune systems by means of attainable physio-immunological approaches. This requires acomprehensive study on the immunophysiological alterations throughout lactation and during mastitis. Thisreview focuses on some factors affecting PMN functions during the lactation cycle and mastitis in highyielding dairy cows.