Lymphocytes recirculate in the bloodstream and lymph flow and pass into the thoracic duct, lymph nodes,
lymphatics, and so-called high endothelial venules (HEV). HEVs enable naïve lymphocytes from the bloodstream
to directly enter a lymph node. The internal organs differ from each other by the presence of HEVs.
HEVs matter much for scanning antigen-rich dendritic cells by naïve lymphocytes before their activation.