High Endothelial Venules

High endothelial venules (HEV) are composed of cuboidal/columnar epithelium and are the major route for lymphocytes to enter the node. HEV contain a large number of aquaporin-1 channels allowing for a large uptake of water which in turn drives lymph flow through the cortex. This fluid is then returned direct to the bloodstream. The venules are the source of most of the node's T cells and B cells and express selectins (receptors for lymphocytes primed with antigens).

HEV express CD34 and GlyCAM-1 which bind to L-selectin on naive lymphocytes. This allows circulating lymphocytes to recognise when they have reached a secondary lymphoid organ and prompts them to leave the bloodstream to enter the lymphatic tissue.