Immunological functions of the mammary gland and its secretion--comparative review.

The mammary gland performs vitally important immunological roles, both in providing passive immune protection to the suckling infant and in immunological defence of its own tissues against infection with microorganisms. These immunological functions differ greatly between species of mammals in both nature and magnitude. In ungulates the mammary gland is singularly responsible for transfer of immunoglobulin (IgG) from mother to young. This process is dependent on a highly selective mechanism which results in the transport of blood-borne IgG molecules across secretory epithelial cells of the colostrum-forming mammary gland and into secretion. Upon ingestion of colostrum by the young ungulate this immunoglobulin is absorbed across the wall of the small intestine and thence into the bloodstream. In other species, including rodents and primates, there is a well-developed local IgA system operating in the mammary gland. In this situation, plasma cells located near the basal membranes of secretory epithelial cells secrete IgA which passes through the epithelial cells and into colostrum of milk. In the species the IgA in mammary secretions is not absorbed into the circulation of the suckling infant; because of its unique property of resisting proteolytic degradation, it may mediate a local protective role in the lumen of the intestine of the suckling infant. Specific immunological protection of mammary tissue may be mediated through blood-derived antibody (particularly IgG), locally synthesized antibody (particularly IgA) or phagocytic cells. Neutrophils arrive in mammary tissue and secretions in very large numbers following bacterial invasion of the gland. It has been established recently that these cells carry cytophilic antibody on their cell membrane. This cytophilic antibody can play an important functional role in enhancing the phagocytic capacity of neutrophils in the mammary gland.