The tight junctions in between epithelial cells play an important role in preventing material from crossing the epithelial sheet. In some intestinal diseases, a cytokine called TNF (for tumor necrosis factor) contributes to the loss of this barrier by disruption of tight junctions. A recent paper shows that TNF induces endocytosis, or internalization, of certain tight junction proteins, which in turn results in barrier loss. Image shows the tight junction proteins occludin (green) and ZO-1 (red) in epithelial cells.
Reference: Image is from the cover of Journal of Cell Biology, April 5, 2010. Amanda M. Marchiando, Le Shen, W. Vallen Graham, Christopher R. Weber, Brad T. Schwarz, Jotham R. Austin II, David R. Raleigh, Yanfang Guan, Alastair J.M. Watson, Marshall H. Montrose, and Jerrold R. Turner, 2010. Originally published in Journal of Cell Bioloy. doi: 10.1083/jcb.200902153. Paper can be found here.