A lymph node is a secondary collections of lymphoid tissue other than the spleen and tonsils situated in the course of lymphatic vessels. Like the spleen it is a dense encapsulated lymphoid organ. Lymph nodes act as filters or traps for foreign particles and antigens. Presentation of antigens to lymphocytes by antigen-presenting cells occur in them. Migration of lymphocytes out of nodes is mediated by the gradient of the glycosphyngolipid, sphingosine 1-phosphate (S1P) which is at higher concentrations externally to lymph nodes and detected by a sphingosine 1-phosphate receptor 1 on the cell membrane of lymphocytes.
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