Function of phagocytes obtained from lacteal secretions of lactating and nonlactating cows

Phagocytes, macrophages and neutrophils, were obtained from lacteal secretions of lactating (n = 13) and nonlactating cows (n = 14). Secretions from nonlactating cows were collected at 7 and 14 days after cessation of lactation. Phagocytes were incubated in vitro with Staphylococcus aureus or Escherichia coli, and function was assessed by fluorescent microscopy of cell suspensions stained with acridine orange and crystal violet. A greater percentage of macrophages from nonlactating cow secretions collected on day 14 phagocytized bacteria than did those collected on day 7. A greater percentage of macrophages from nonlactating cow secretions collected on days 7 and 14 phagocytized bacteria than did neutrophils obtained from the same secretions. A similar percentage of phagocytes from nonlactating cow secretions phagocytized bacteria, compared with phagocytes from lactating cow secretions. Results indicated that the intramammary macrophage may be most important in defense of the mammary gland during the early nonlactating period, because it was more phagocytic than the neutrophil and was more active at 14 days than at 7 days into the nonlactating period.