Collection of Milk Somatic Cells and Blood Leukocytes for Gene ExpressionFifty milliliters of milk was collected from each gland at 0, 4, 8, 12, 24, 36, 48, and 72 h relative to the challenge for analysis of gene expression in milk somatic cells. The milk samples collected at 12-h intervals were collected before normal milking. An additional 50 mL of milk was collected at 0 and 8 h for analysis and separation of macrophages and neutrophils. Milk samples were collected into 50-mL conical tubes before full milk withdrawal and transported to the laboratory on ice. Samples were centrifuged at 1,500 × g for 20 min to pellet the cells. The cell pellet was then resuspended in 10 mL of PBS and centrifuged again to wash the somatic cells. The cells were placed in Trizol Reagent (Life Technologies, Carlsbad, CA) and stored at −80°C for later RNA isolation. Blood was sampled from coccygeal vessels into 8-mL lithium heparin evacuated tubes (Vacutainer, Becton Dickinson, Franklin Lakes, NJ) at 0 and 8 h relative to the treatment for isolation of monocytes and neutrophils.