The immunologic significance of the mammary gland

The immunologic role of the mammary gland is to transfer ready made antibodies, the 'maternal immunologic endowment', from the maternal serum to the gastrointestinal tract and eventually the blood stream of the immunologically naive infant. In certain species, the mammary gland deputizes for the fetal membranes which are incapable of transmitting antibodies, and in others it continues a process initiated in utero. Antibody secretion by the mammary gland is highly selective. Certain antibodies are preferentially taken up from the serum and concentrated. Absorption from the host's gastrointestinal tract is equally selective and of short duration. In man, gastrointestinal antibody absorption does not occur. Despite this, secretory IgA antibodies fulfil an immunoprotective role within the lumen of the gut, acting against a variety of enteric microorganisms. These antibodies are synthesized by plasma cells associated with active mammary tissue. Apart from their beneficial roles, mammary glands may exert an inimical role by virtue of the nature of their exosecretions. In some species they transmit maternal isoantibodies that can cause hemolytic disease of the newborn. They secrete certain proteins, such as casein, which are a potent source of allergic reactions, possibly including the 'sudden infant death syndrome'. Finally, viable leukocytes are a largely neglected but constant ingredient of colostrum and milk. These can reach the circulation of the recipient and can interact with mononuclear cells of the host; as a result, transplantation immunity, tolerance, and graft versus host disease may develop in some species.