上皮細胞や内皮細胞が外部環境からの異物・病原体・有害物質の侵入を防ぎつつ、必要な物質だけを選択的に透過させる生理的な機能を指す。この機能は、細胞間接着構造（タイトジャンクションなど）や細胞極性の維持によって成立する。

This refers to the physiological function by which epithelial and endothelial cells prevent the entry of foreign substances, pathogens, and harmful substances from the external environment while selectively allowing only necessary substances to pass through. This function is maintained through intercellular adhesion structures (such as tight junctions) and the maintenance of cellular polarity.