White blood cells in milk are known as somatic cells, and the number per ml is called the somatic cell count or SCC. White blood cells are important for fighting bacteria. The three main types of white blood cells are the macrophages, neutrophils and lymphocytes. These cells are important for the immune response.

Macrophages are always present in milk and are responsible for the 20-40,000/ml SCC of healthy cows. Neutrophils and lymphocytes are responsible for the massive increase in SCC associated with infection.

High bulk milk SCCs mean that high numbers of cows are infected within the herd. (at >300,000 cells per ml approximately 40% of cows will have an infection). Individual cow SCCs are an indicator of which cows are infected in the herd. SCCs can be used to make day to day management decisions; whether to dry off low yielding and high SCC cows early; assist with decisions on dry cow treatment strategy.