How does IgA get from one side of the luminal surface to the other?
and where does it's final destination?
IgA is made in mucosa-associated lymphoid tissue (MALT) and is transported across the barrier of the mucosal epithelium. First, IgA binds to the poly-Ig receptor on the basolateral surface of an epithelial cell, followed by uptake through receptor-mediated endocytosis into an endocytic vesicle. On reaching the opposite face of the cell, the apical surface, the vesicle fuses w/ the membrane. Here the poly-Ig receptor is celaved proteolytically between the membrane-anchoring and the IgA-binding regions, thus releasing IgA into the mucous layer on the surface of the epithelium. IgA remains attached to a small piece of the poly-Ig receptor, called the secretory component, which holds the IgA at the epithelial surface through interactions with molecules in the mucus. The rest of the poly-Ig receptor is degraded and serves no further purpose.
IgA is released into the lumen of the GI, urogenital, and resp. tracts, onto surface of the eyes, into the nose and throat, and into breast milk (which is the route by which newborn babies receive protective maternal IgA.