The leukocyte as a defense mechanism.

Within the udder, neutrophil polymorphonuclear (PMN) leukocytes are capable of phagocytosing a wide variety of particles (microorganisms, milk fat globules, casein). Studies of PMN leukocyte function indicate that milk fat globules and casein are perhaps significant deterrents to phagocytosis and destruction of Staphylococcus aureus by PMN leukocytes in milk. These deterrents could be a major factor in establishment of intramammary infections by invading mastitis pathogens. Elimination of fat and casein from milk during clinical or subclinical mastitis may free PMN leukocytes to function properly and may remove infection from the mammary gland. Further observations on PMN leukocyte function indicate that considerable variation exists among cows in the ability of whole milk and skimmed milk to support phagocytosis and in the ability of PMN leukocytes to phagocytose. Variation in this basic defense mechanism could account for the differences that exist in susceptibility to infection by mastitis pathogens.