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.
of the node. The sinuses have a lining of endothelium that is continuous where it is directly adjacent to the connective tissue of the capsule or trabeculae but discontinuous where it faces the lymphatic parenchyma. Although a macrophage may reside in the lymphatic parenchyma, it often sends pseudopods (long cytoplasmic processes) into the sinus through these endothelial discontinuities. These pseudopods monitor the lymph as it percolates through the sinus.
Lymphatic sinuses are not open spaces, as are blood sinuses. Particularly in the medulla, macrophage processes, along with the reticular fibers surrounded by reticular cell processes, span the lumen of the sinus and form a crisscrossing meshwork that retards the free flow of lymph and enhances its filtration. Antigenic material and transformed cells of metastatic cancer are trapped by this mechanical filter and then phagocytosed by macrophages. In metastatic cancer, the system can be overwhelmed by an excessive number of cancer cells flowing through the lymphatic sinuses; as a result, the cells may establish a new metastatic site in the lymph node.
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