Transcytosis

In the examples discussed above receptors were internalised from the apical surface of the cell and returned to there after unloading their ligand in the endosome for recycling. However, in some cases the proteins are send to the basolateral surface instead. For example, antibodies in milk bind to specific receptors in the apical (luminal) surface in the intestine of a suckling, are internalised with their receptor in CCV and transported to early endosomes. There the receptor-antibody complex is packed into transport vesicles which fuse with the basolateral surface of the cell. The neutral pH of the extracellular fluid leads to receptor-ligand dissociation and the antibody is released into the sucklings blood stream.

In the mother the transport occurs by a similar mechanism, but in the opposite direction: from blood antibodies are bound to the basolateral surface of the epithelium in the milk gland and transported to the apical surface and into the milk.

Epithelial cells have different early endosomes for their apical and basolateral surfaces, these are linked by a common late endosome.

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