Although limited paracellular passage of immunoglobulins may occur during inflammation (mastitis), uptake and transport of immunoglobulin across the mammary epithelial barrier is thought to occur primarily through an Fc-receptor-mediated process [1, 7, [118] [119] [120] . Immunoglobulins are thought to bind to receptors at the basolateral surfaces of the mammary epithelial cell. These receptors are specific for the Fc portion of the immunoglobulin molecule. The receptor-bound immunoglobulin is internalized via an endocytic mechanism [121] , transported to the apical end of the cell and released into the alveolar lumen. Recent studies have shed additional light on the details of this process [122] .