Relationships between the phagocytic ability of milk macrophages and polymorphonuclear cells and somatic cell counts in uninfected cows

Phagocyte numbers and activities were compared in milk from 2 groups of uninfected mammary-gland quarters from 3 cows each: 6 quarters with a high (>= 200 000/mL) somatic cell concentration (SCC), analyzed as 4 individual quarters and 1 pooled sample; and 12 quarters with a low SCC (< 200 000/mL), analyzed as 6 paired samples. The concentrations and ability of macrophages and polymorphonuclear (PMN) cells to phagocytize fluorescent microspheres were determined by flow cytometry after exposure of the cells to the microspheres. The macrophages and PMNs contained 2 major subpopulations, characterized by low phagocytic (LP) or high phagocytic (HP) ability. The quarters with high SCCs had significantly lower percentages of HP cells than did the quarters with low SCCs (P < 0.01). Whether mammary-gland quarters or cows were the unit of analysis, the HP/LP ratio was negatively related to the SCC (P < 0.04), which explained more than 50% of the SCC variability. Thus, poor bovine mammary-gland phagocytic ability may be associated with high SCC. Longitudinal studies are suggested to further explore and characterize these relationships.