Tight junction.

The protein seal between adjacent cells that controls what passes between them.

Tight junctions are protein complexes that seal the space between adjacent epithelial or endothelial cells, controlling what can pass between cells rather than through them. They are built from proteins including claudins, occludin, and the zonula occludens family, and they form the structural basis of biological barriers: the intestinal lining, the blood-brain barrier, and the barriers of the kidney and skin. They are not static seals. Tight junctions are dynamically regulated, opening and closing in response to signals, which is how the gut can absorb nutrients while still excluding bacterial products, and this regulated permeability is normal physiology rather than a defect. The term appears in peptide contexts in two places. Gut-barrier research concerns compounds proposed to influence intestinal tight-junction integrity, and drug-delivery research concerns the deliberate, temporary opening of tight junctions as a way to get large molecules across a barrier they cannot otherwise cross, which is one approach to the oral peptide problem. That second use highlights the tension: any mechanism that loosens a barrier to let a drug through also loosens it to whatever else is present. Barrier permeability is a controlled variable in the body, and more open is not a health goal in itself.