Cells in local reaction to experimental Staphylococcus aureus infection in bovine mammary gland

Blood cells infiltrating secretory parenchyma of bovine mammary glands experimentally infected with Staphylococcus aureus were studied. Quantitative cytology demonstrated lymphocytes, plasma cells, monocytes, macrophages, and neutrophils more numerous in infected than control quarters and more prevalent at 10 than 2 days postinfection. These cells preferentially infiltrated the zone of the infected quarter closest to the gland cistern. Lymphocytes tended to remain associated with the epithelium, and many were within spaces between swollen secretory cells. Monocytes entered between alveolar epithelia and appeared to mature into macrophages and migrate to lumina where they contained milk constituents, degenerate neutrophils, and cocci. Neutrophils were observed primarily in alveolar lumina where many degenerated. Some neutrophils in various stages of degeneration also were lodged in the alveolar epithelium. Plasma cells were prevalent in infected tissue stroma, and most contained Immunoglobulin A which became more numerous as infection progressed. Our observations provide quantitative and ultrastructural information on cell types in bovine mastitis and are consistent with the concept of a cellular immune response to Staphylococcus aureus invasion.