The Lymphatics

• Lymph Node Function:

  Deal with antigens.

  Uptake and processing of antigen which leads to its destruction.
• Location:

  Found throughout the body especially numerous in areas draining organs with environmental contact.
• Mostly Found:

  Respiratory and digestive tracts have large groups of draining lymph nodes (hilar, mediastinal and mesenteric).

  The skin also has groups of lymph nodes draining one or several areas (cervical, inguinal and axillary).
• Lymph Node Anatomy

  System of draining lymph node groups, converge into a single lymphatic vessel, the thoracic duct.

  The duct drains into the bloodstream, creating an effective filtering system for purging unwanted substances
• The immune system reacts to some classes of antigens differently than it does others:

  Structure

  • Small and round
  • Do not exceed 1cm in diameter
  • During immune reactions they can become larger
  • Cut surface is pinkish in colour and homogenous

  
  

  Apoptosis plays an important role, the immune system removes cells that are redundant or potentially dangerous. Only cells producing antibodies are selected to survive and go on to produce antibodies or become memory cells
• Lymphatics

  • Afferent lymphatics enter the lymph node in the sub-capsular sinus
  • The sinus has an endothelial layer
  • System of branching sinuses that arise from it, and eventually drains into the efferent lymph vessels in the hilus, has no endothelial lining
• Veins

  • Venous drainage follows the arteriolar route, and the vein leaves the lymph node via the hilus
  • Post-capillary venules have a specific function in the lymph node because they are the gate of entry of circulating lymphocytes into the lymph node
• Arteries

  • Enter the lymph node at the hilus
  • Branch and form a capillary network in the parenchyma, both in the para-cortex and the follicles
  • Subtle differences in the basal membrane of the capillaries at these 2 locations exist.
  • These compartment differences probably play a role in positioning lymphocytes in the different compartments
• Structure of the Lymph Node

  4 compartments:

  1. Follicles
  2. Paracortex
  3. Medullary cords
  4. Sinuses
• Function of the Follicle - germinal centre cell reaction leading to the formation of precursors of antibody forming cells and of memory B-cells

  
  

  Primary follicles are aggregates of small, dark staining lymphoid cells. When a germinal centre develops in a primary follicle it becomes a secondary follicle.

  Lymphoid cells, dendritic reticulum cells and macrophages are all present in the follicle centre.
• Medullary Cords - plasma cell reaction leads to formation of antibody secreting B-cells
• Paracortex - specific cellular response takes place, generating antigen specific T-cells and memory T-cells

  The paracortex is easily recognisable because of its location and its typical structural elements:

  1. Epithelioid venules
  2. Interdigitating cells
• Sinuses - macrophages clear the lymph through which the antigens pass to the lymph node
• The lymphoid cells of the primary follicle are B-cells and there are 4 distinct types of B-cells present:

  • Centroblast
  • Centrocytes
  • Lymphoblasts
  • Other Lymphoid Cells -> can include plasma cells, immunoblasts as well as small lymphocytes which are probably T-cells
• Centroblasts: large cells with a large nucleus and 1-3 prominent nucleoli
• Centrocytes: both large and small cells. The large cells have a medium - large nucleus with inconspicuous nucleoli. The small cells have small, irregular nucleus that may be elongated.
• Lymphoblasts: these cells are scarcer than the other cell types.

  • They have a medium sized nucleus with inconspicuous or no nucleoli and very little but intensely basophilic cytoplasm.
• Dendritic reticulum cells (DRC) trap antigens on their surface, process them and present them to B-cells.

  DRC are difficult to recognise in the H&E section but have been characterised firstly by electron microscopy and later by IHC.

  Medium-large nucleus and an invisible cytoplasm. Using IHC, the cytoplasm shows many long and slender protrusions.

  The macrophages are large cells with abundant cytoplasm containing phagocytised debris and apoptotic bodies.
• Epithelioid Venules

  These distinctive vessels are only found in the paracortex. They are lined with plump, cuboidal endothelial cells. They function as a gate of entry for lymphocytes from the peripheral blood.
• Interdigitating Cells

  These are large cells, with a large bizarre nucleus. The cytoplasm is abundant and clear with ill-defined borders.

  Their function is to present antigens to lymphoid cells and thus initiate an immune response.
• T-Lymphocytes

  Most of the paracortical cells are small T-cells. These are a mix of helper and suppressor T-cells.

   
• Medullary Cords

  Found in the hilar region of the lymph node, between the sinuses.

  Composed of:

  • Lymphoid cells
  • Small lymphocytes make up majority of cells in medullary cords. They have small round nuclei, scant cytoplasm and are T-cells
  • Macrophages
  • Somewhat scarce and not avidly phagocytic due to function being antigen presentation not phagocytosis.
  • Mast cells
  • Most frequent in lymph node in medullary cords
• Sinuses carry the lymph from afferent lymphatics through the node to the efferent lymph vessels

  The sus-capsular sinus is partially lined by endothelial cells while the medullary sinuses lose their endothelial lining and acquire a lining of macrophages

  Small lymphocytes can be found in the sinuses.

   
• Immunohistochemistry

  Used primarily for demonstrating the presence/absence of clonality, as well as the presence/absence of normal compartmentalisation.

  All compartments have a specific function therefore have a typical immune profile.

   
• Cluster of Differentiation Markers

  • Surface molecules that serve as useful markers of cellular identity.
• IHC -> Be Friendly = B cells/Follicle

  • The follicle is a B cell region. B cells are labelled with antibodies to the B cell surface antigens, CD10, CD19, CD20 and CD22.
  • Ig's are present, mostly IgM but also small amounts of IgG and IgA.
  • Any T cells present will stain with CD4 (T-helper) and CD8 (T-suppressor) in a ratio of about 3:1
  • Macrophages stain with anti-chymotrypsin and also with the surface antigens CD11b, CD35 and CD68
• Paracortex - IHC = Think Positive - T cells/Paracortex

  • The CD4:CD8 ratio is 2-3:1. also express all or most mature T cell markers, such as CD2/3/5 and CD7. (lower numbers = T cell)
  • Endothelial cells of the venules stain with a specific antibody, HECA 452
  • The interdigitating cells can be recognised using an S-100 antibody as well as HLA-DR and CD24
• Medullary Cord - IHC

  Predominantly B cell areas - fair number of CD4 T helper cells present

  Kappa and Lambda light chain cells will be found in ratio of 2:1

   
• Sinus - IHC

  Sip Martinis - Sinus/Macrophages

  IHC shows the macrophages to be positive:

  • Lysozyme
  • A1-antitrypsin
  • CD68
• Acute Nonspecific Lymphadenitis


  Lymph nodes undergo reactive changes whenever they are challenged by microbiologic agents, cell debris or foreign matter introduced into wounds or into the circulation

  clinically, nodes with acute lymphadenitis are enlarged and tender
• Chronic Nonspecific Lymphadenitis


  lymph nodes in chronic reactions - not tender.

  common in the inguinal and axillary nodes

  lymphadenopathy -> increased size of lymph nodes
• Lymphoma = lymphoid neoplasm - discrete tissue mass

  1. Non-Hodgkin Lymphoma
  2. Hodgkin Lymphoma

  2/3's of NHL and all HL present with Non-tender nodal enlargement - localised or generalised.
• WHO Classification of lymphoid neoplasms

  1. precursor B-cell neoplasms
  2. Peripheral B-cell neoplasms
  3. Precursor T-cell neoplasms
  4. Peripheral T-cell and NK-cell neoplasms
  5. Hodgkin Lymphoma - neoplasms of Reed-Sternberg cells
• Histopathology

  • antigen receptor gene rearrangement precedes transformation
  • vast majority of neoplasms are of B-cell origin -> remainder are T-cell tumors.
  • neoplasms disrupt normal architecture and function of immune system
  • Hodgkin spreads in orderly fashion -> NHL less predictable
• Sentinel Lymph Node Examination

  based on theory that these tumors spread in orderly fashion from primary site to nearest lymph node - sentinel lymph node.

  detected by use of radioactive dye injected at site of malignant melanoma or tumour in breast and then excision -> accumulation of blue dye.

  • radioactive isotope = Technetium-99 -1 day half-life
• IHC used:

  For malignant melanomas, S-100 or Melin A are used.

  For invasive breast carcinomas, an antibody to cytokeratin is used. The best antibody is AE1/AE3