High endothelial venules (HEV) are specialized post-capillary venous swellings characterized by simple cuboidal cells as opposed to the usual endothelial cells found in regular venules. HEVs enable lymphocytes circulating in the blood to directly enter a lymph node (by crossing through the HEV). In humans, HEVs are found in all secondary lymphoid organs (with the exception of spleen, where blood exits through open arterioles and enter the red pulp), including hundreds of lymph nodes dispersed in the body, tonsils and adenoids in the pharynx, Peyer's patches (PIs) in the small intestine, appendix, and small aggregates of lymphoid tissue in the stomach and large intestine...