• 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