- 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+)
- Site of B-cell localization and proliferation.
- 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