Transcytosis refers to a receptor-mediated, vesicle-bound transport of substances through the cell. Here, the substance to be transported is taken up endocytotically into the cell. They are transported through the cell with their receptor molecules via"early endosomes", "late endosomes" and finally transport vesicles. At the opposite side of the cell, the transport vesicles again fuse with the cell membrane and release their "cargo " into the extracellular space.
Transcytotic transports include transferrin (iron transport), lipoproteins, hormones (insulin), immunoglobulins (IgA), microorganisms, toxins (e.g. botulinum toxin) through cells.
Transcytosis occurs mainly in epithelial cells (kidneys, intestines, etc.) whose intercellular spaces are closed by "tight junctions". Furthermore in endothelia, but also in bone (osteoclasts), in M-cells of Peyer's plaques and in nerve cells.