Diagram of a follicular dendritic cell. This cell, usually found in germinal centers, has multiple, thin, hair-like cytoplasmic processes that interdigitate between B lymphocytes. Antigen-antibody complexes adhere to the dendritic cytoplasmic processes by means of Fc receptors. Follicular dendritic cells are not antigen-presenting cells because they lack MHC II molecules.
Specialized high endothelial venules (HEVs) are the site of entry for circulating lymphocytes into the lymph node
In addition to lymph, lymphocytes also circulate through the lymph nodes. Although some lymphocytes enter nodes through afferent lymphatic vessels as components of lymph, most enter the node through the wall of postcapillary venules located in the deep cortex (see Fig. 13.22). Because the postcapillary venules are lined by cuboidal or columnar endothelial cells, they are referred to as high endothelial venules (HEVs) (Fig. 13.23). These specialized endothelial cells possess receptors for antigen-primed lymphocytes. They signal lymphocytes to leave the circulation and migrate into the lymph node. Both B and T cells leave the bloodstream through HEVs, crossing the endothelium
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