Transcytosis, or vesicle transport, is one of three mechanisms that facilitate capillary exchange, along with diffusion and bulk flow.

Substances are transported through the endothelial cells themselves within vesicles. This mechanism is mainly used by large molecules, typically lipid-insoluble preventing the use of other transport mechanisms. The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side. Vesicles are capable of merging, allowing for their contents to mix, and can be transported directly to specific organs or tissues.

Pathology

Due to the function of transcytosis, it can be a convenient mechanism by which pathogens can invade a tissue. Transcytosis has been shown to be critical to the entry of Cronobacter sakazakii across the intestinal epithelium and the blood-brain barrier.

Listeria monocytogenes has been shown to enter the intestinal lumen via transcytosis across goblet cells. Shiga toxin secreted by entero-hemorrhagic E. coli has been shown to be transcytosed into the intestinal lumen. These examples illustrate that transcytosis is vital to the process of pathogenesis for a variety of infectious agents.

Transcytosis in Pharmaceuticals

Pharmaceutical companies are currently exploring the use of transcytosis as a mechanism for transporting therapeutic drugs across the human blood-brain barrier. Exploiting the body's own transport mechanism can help to overcome the high selectivity of this barrier, which blocks the uptake of most therapeutic antibodies into the brain and central nervous system.