Immunophysiology of the mammary gland and transmission of immunity to the young.

The nature and extent of the transmission of immunity by the mammary gland from the mother to her offspring is related to the rate of transmission during foetal life: in mammals with immunoglobulin (Ig) permeable placenta, the systemic immunity of the mother is transferred during foetal life; in contrast, in artiodactyls with Ig impermeable placenta, systemic immunity is transmitted just after birth by the Ig enriched colostrum, and the neonate gut is permeable to Ig during the first 36 h after birth [1]. Then the transmission of mucosal immunity takes over systemic immunity. The nature and purpose of both types of immunity are different: the systemic immunity is convoyed by the Ig isotype with destroying properties in the presence of the complement, IgM and more appropriately by IgG (of lower molecular weight and thus more prone to diffuse in tissue spaces and trap the pathogen) [2]; in contrast, the mucosal protection is assured by dimeric IgA, which protects the mucosae by a non inflammatory mechanism such as the inhibition of pathogen attachment to the mucosae: in simplestomached animals, both colostrum/milk are rich in secretory dimeric IgA (S-(IgA) 2 ): moreover, colostrum is richer in IgA than milk in species with systemic immunity protection during fetal life. Since secretory IgA in mammary secretions are specific of pathogens in contact with the mucosae, the mother thus ensures an immunity of the newborn, until weaning, at a time when the mucosal immune system of the young is able to mount a good protective immune response [3]. In the sow as in the mouse, Ig plasma cells are present in the mammary gland at delivery and during lactation [4, 5]. In the mouse, it has been shown conclusively that these plasma cells originate as cell-precursors in the gut Peyer’s patches then they migrate in the blood and home into the mammary gland (the so-called entero-mammary link) at a time when this gland begins lactation [6]. Once in the mammary gland these plasma cells release in situ the dimeric IgA which will be excreted in the milk after transcytosis of the mammary epithelial cell via the IgA receptor. In species such as the mouse, the concentration of IgA in the milk increases during lactation and is accounted