From the NMC Newsletter "Udder Topics", October 1996

Rapid movement of leukocytes into the mammary gland is one of the most important natural defense mechanisms against mastitis. Leukocytes, also called somatic cells, are present at concentrations less than 200,000 cells/ml in milk from uninfected glands, and are the majority of cells comprising the somatic cell count (SCC). Leukocyte numbers increase markedly in response to invading pathogens, and may reach concentrations of millions/ml in acute mastitis cases.

The vast majority of leukocytes in milk during inflammation are the polymorphonuclear neutrophilic leukocytes (PMN) which enter the mammary gland from the blood. The PMN are attracted to the quarter by the presence of infecting microorganisms. The PMN recognize bacterial cells coated with antibodies, engulf these microorganisms, and kill them - a process known as phagocytosis. A 12 to 24-hour lag after initial infection generally occurs before a high leukocyte concentration is found in milk, although some microorganisms such as Eschericia coli cause a more rapid response.

Other leukocyte types in the udder are lymphocytes and macrophages. Lymphocytes are the "conductors" of the immune system, and coordinate the activities of all cells in the immune system by releasing soluble mediators called cytokines. Cytokines are hormone-like proteins that are produced locally in the udder in very low levels. These proteins play an important role in stimulating the recruitment of cells into the udder following bacterial invasion. Cytokines also are responsible for initiating the development of antibody-producing plasma cells from activated lymphocytes.

Like the PMN, macrophages can phagocytize and kill bacteria. However, a major role of macrophages is their ability to stimulate the migration of PMN into milk by releasing cytokines and leukotrienes.

Source: "Current Concept of Bovine Mastitis", National Mastitis Council (1996).