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Summary
: The lymph node cortex is a critical site for encounter between recirculating T cells and their specific antigens. Due to its extreme plasticity, little is understood of the underlying functional unit of the lymph node cortex, the paracortical cord. The idealized paracortical cord (approximately 100 nm by 1000 μm) stretches from a medullary cord to the base of a B‐cell follicle. In cross‐section, a cord can be visualized as a set of nested cylinders consisting of spaces bounded by cells. The spaces are: i) the lumen of the high endothelial venule (HEV), ii) perivenular channels ‐ narrow potential spaces (0.1 μm) tightly encircling the HEV, iii) corridors – broad spaces (10–15 μm) constituting the majority of the parenchyma, and iv) the cortical sinus. In addition to these spaces for cell traffic, the conduit (fifth space) is a special delivery system for the transit of soluble factors to the HEV and emigrating lymphocytes. The cellular barriers between these spaces are high endothelium, tibroblastic reticular cells, or sinus‐lining cells. This review describes the spaces of the paracortical cord and their cellular boundaries, outlines the movement of cells and fluids through these spaces, and discusses how this anatomy affects the efficiency of surveillance by T cells.