How the Immune System Works by Lauren M. Sompayrac

Author:Lauren M. Sompayrac [Sompayrac, Lauren M.]
Language: eng
Format: epub, pdf
Tags: Health & Fitness, Diseases, Immune System
ISBN: 9781405162210
Publisher: Wiley-Blackwell
Published: 2008-03-25T00:00:00+00:00

HIGH ENDOTHELIAL VENULES

A second anatomical feature common to all secondary lymphoid organs except the spleen is the “high endothelial venule” (HEV). The reason HEVs are so important is that they are the “doorways” through which B and T cells enter these secondary lymphoid organs from the blood. Most endothelial cells that line the inside of blood vessels resemble overlapping shingles which are tightly “glued” to the cells adjacent to them to prevent the loss of blood cells into the tissues. In contrast, the small blood vessels that collect blood from the capillary beds (the postcapillary venules) within most secondary lymphoid organs are lined with special endothelial cells that are shaped more like a column than like a shingle.

These tall cells are the high endothelial cells. So a high endothelial venule is a special region in a small blood vessel (venule) where there are high endothelial cells. Instead of being glued together, high endothelial cells are “spot welded.” As a result, there is enough space between the cells of the HEV for lymphocytes to wriggle through. Actually, “wriggle” may not be quite the right term, because lymphocytes exit the blood very efficiently at these high endothelial venules: about 10 000 lymphocytes exit the blood and enter an average lymph node each second by passing between high endothelial cells.

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