Mastitis is caused by bacterial invasion into the udder.
The small numbers of somatic cells that are normally
present in milk attempt to resolve this intramammary
infection immediately. The cellular presence in milk is one
of the important protective mechanisms of the mammary
gland and may be considered as a surveillance function in
the uninfected gland. Both bacteria and leukocytes in the
infected quarters release chemo-attractive products for
leukocytes, especially neutrophils.