Lymphoid Structures

  • Lymph node:  A 2° lymphoid organ that has many afferents, 1 or more efferents. Encapsulated, with trabeculae. Functions are nonspecific filtration by macrophages, storage of B and T cells, and immune response activation.
  • Follicle
    • Site of B-cell localization and proliferation.
      • In outer cortex. 1° follicles are dense and dormant.
      • 2° follicles have pale central germinal centers and are active (plasma cells with germinal center inside, corona outside).
      • Surrounded by the mantle and the margin
      • Start proliferating, producing antibodies after stimulation by antigens, undergo isotype switching from IgM to other types of immunoglobulins
      • Defective in Bruton agammaglobulinemia (cannot produce immunoglobulins)
      • Both paracortex and follicle are defective in SCID
    • Reticular cells produce collagen type III
    • Macrophages (APCs)
      • Phagocytic
      • Antigen Presenters
    • Dendritic cells
      • Less phagocytic
      • MHC CII
    • Enlargement in rheumatoid arthritis and early HIV (infection of dendritic cells, which are CD4+)
  • Paracortex
    • Houses T cells. Region of cortex between follicles and medulla. Contains high endothelial venules through which T and B cells enter from blood.
    • Not well developed in patients with DiGeorge syndrome.
      • DiGeorge syndrome – thymic aplasia, T-cell deficiency with poor activity vs viral, fungal, intracellular organisms
    • Paracortex enlarges in an extreme cellular immune response (eg, viral infection).
    • Reticular cells produce collagen type III
    • Both paracortex and follicle are defective in SCID (related to adenosine deaminase deficiency)
    • Enlargement in viral infection (such as infectious mononucleosis)
  • Medulla
    • Consists of medullary cords (closely packed lymphocytes and plasma cells) and medullary sinuses. Medullary sinuses communicate with efferent lymphatics and contain reticular cells and macrophages.
  • Enlargement of sinus histiocytes when LN draining tissue with cancer.
  • Lymph flow
    • Lymph from afferent lymphatics via subcapsular sinuses → convex surface → reticular/cortical sinuses get APCS resulting in T and B cell activation →  T cell proliferation & Effector B-cell antibody production → medullary sinuses → efferent lymphatics
    • Site of flow of malignancies
  • Blood flow
    • Artery in lymph node hilum → capillaries → Postcapillary venule expresses homing in receptors to catch lymphocytes → efferent lymphatic duct → major/thoracic duct → IJV/subclavian vein
  •  Lymphadenopathy
    • Painful LAD – acute infection
    • Painless LAD – chronic inflammation, metastatic carcinoma, or lymphoma

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