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.
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.