FUNCTION OF SOMATIC CELLS
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.
The neutrophil polymorphonuclear (PMN) leukocytes
are the second line of defense against mammary gland
infection. PMN’s are phagocytic cells which engulf and kill
bacteria. However, in bovines, the phagocytic ability of
PMN of milk can consume milk fat globules and casein
(Opdebeeck, 1982) leading to putrefaction of milk. An
inflammatory response is usually initiated when bacteria
enter the mammary gland through the teat canal and
multiply in the milk. Although bacterial toxins, enzymes
and cell-wall components have a direct effect on the
function of the mammary epithelium, they it also stimulate
the production of numerous mediators of inflammation,
mainly neutrophils (Gallin et al., 1992), due to edema,
vasodilation and increased vascular permeability (Nonnccke
and Harp, 1986).