FIGURE 23-11 Transcytosis of IgA and IgG. (a) IgA, found in tears and in the secretions of various mucous membranes, must be transported across the epithelium. IgA binds to the polymeric IgA receptor and is endocytosed. As the resulting complex is transported across the epithelial monolayer, a portion of the receptor is cleaved, and the IgA, still bound to a portion of the receptor, the secretory piece, is released at the apical side. (b) Suckling rodents acquire Ig from their mother’s milk. At the apical surface of its intestinal epithelium, the newborn possesses the neonatal Fc receptor (FcRn), whose structure resembles that of class I MHC molecules (see Figure 23-23). After this receptor binds to the Fc portion of IgG, transcytosis moves the acquired IgG to the basolateral side of the epithelium. In humans, the syncytial trophoblast in the placenta expresses FcRn and so mediates acquisition of IgG from the maternal circulation and its delivery to the fetus (transplacental transport).