en:lymph_node [Chapters]

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====== LYMPH NODE ======

  * Afferent lymph vessels:
    * Penetrate capsule, enter marginal sinus, communicate with intranodal sinuses, then become efferent vessels, which lack an endothelial lining
    * Intranodal vessels contain littoral cells or histiocytes with phagocytic properties
  * Capsule:
    * Thin fibrous connective tissue covering of lymph node
    * May be thicker at hilus
    * Connected to fibrous trabeculae which penetrate the node
    * Capsule may contain smooth muscle cells (Anat Rec 1975;183:517)
  * Cortex:
    * Subcapsular portion of node with largest number of follicles (primary or secondary)
  * Primary follicle:
    * Round aggregates of small, dark staining inactive (naïve) B lymphocytes, usually near the capsule, within a network of follicular dendritic cell processes
    * No germinal center present
  * Secondary follicle:
    * Arises from primary follicle that develops germinal centers (see below) due to antigenic stimulation of B cells and production of antibodies
    * Contains pale staining germinal center which may be polarized towards site of antigen entry
    * Surrounded by mantle zone and marginal zone lymphocytes
    * Germinal center:
      * Contains predominantly B lymphocytes (including centroblasts and centrocytes) and scattered follicular T helper cells and T regs
      * Also tingible body macrophages and follicular dendritic cells
    * Mantle zone:
      * Tightly packed small B lymphocytes of the primary follicles, pushed aside by the germinal centers
    * Marginal zone:
      * Less packed small B lymphocytes with more cytoplasm
      * Light zone on outer rim of mantle zone
      * Contains a mix of post-follicular memory B cells derived after stimulation of recirculating cells from T cell dependent antigen and naïve B cells
      * Often not well developed in lymph nodes
  * Paracortex:
    * Tissue between cortical follicles and medulla (see below)
    * Contains predominantly dark staining mature T cells, B immunoblasts, interdigitating dendritic cells, plasmacytoid dendritic cells, histiocytes and high endothelial venules (postcapillary venules lined by plump endothelial cells that express leukocyte adhesion molecules and contain intraluminal lymphocytes)
    * Expands during cell mediated immunological reactions
    * Has coarse network of reticulin fibers
  * Medulla:
    * Portion of node closest to hilum
    * Contains the medullary cords, sinuses and vessels but minimal number of follicles
  * Medullary cords:
    * Found in hilar region between the sinuses, composed mostly of small B and T lymphocytes, plasmacytoid lymphocytes, plasmablasts and plasma cells
  * Sinuses:
    * Carry lymph from afferent to efferent lymphatics
    * Subcapsular sinus is below capsule and partially lined by endothelium
    * Becomes "medullary" as it approaches the hilum and is lined by macrophages
    * Also contains mast cells and plasma cells
  * Vessels:
    * Blood enters and leaves lymph node at hilus

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