Usually lower than 1 × 105 cells/mL milk. However, a SCC higher than 2 × 105 cells/mL milk is considered to be a more practical distinguishing threshold for IMI.
SCC is greater than 2 × 105 cells/mL milk according to severity of IMI; with severe IMIs, the SCC may reach 1 × 106 cells/mL milk or more within a few hours.
Leukocytes
75% of SCC.
Dramatic increase occurs according to severity of IMI at early stages due to recruitment of immune cells from the marginal pool and bone marrow into the MG environment.
Macrophages
35–79% of total leukocytes in milk, constituting the predominant cell type.
9–32% of total leukocytes in milk.
Lymphocytes
10–28% of total leukocytes in milk. The proportions of T- and B-lymphocytes in milk are approximately 40–50% and 20–25%, respectively. T-cells prevail and are predominantly CD8+ subset with memory characteristics (comprising approximately 50–60% of the T-lymphocyte population).
14–24% of total leukocytes in milk. CD4+ T-cells become the predominant activated phenotype in response to recognition of Ag-MHC class II complexes on Ag-presenting cells, such as B-cells or macrophages. In some circumstances, such as chronic Staph. aureus IMIs, CD8+ are predominantly recruited compared over CD4+ T-lymphocytes.
PMNs
3–26% of total leukocytes in milk.
The predominant cell type, constituting up to 90% of the total milk leukocytes or more. With chronic bacterial IMIs, PMNs also remain as the predominant cells, even for months.
