Lymphoid anatomy

• Describe the lymphoid organs

  The lymphoid organs can be split into:

  • Primary lymphoid tissues: this is where B and T cells develop, such as the thymus and bone marrow
  • Secondary lymphoid tissues: this is where lymphocytes migrate to, such as the lymph nodes, spleen and MALT (mucosa associated lymphoid tissues)
• Describe how lymph is pumped around the body

  Lymph is not pumped by the heart muscles and valves, but rather in the following mechanism:

  1. Lymph drains from the lymph capillaries into the lymph nodes via afferent lymphatic vessels.
  2. Efferent vessels contain valves, which can close to great pressure gradients along the vessels.
  3. This, coupled with contraction of the skeletal muscle walls, leads to movement of lymph along the vessels.

  
• Describe the cycle of lymphocyte activation

  1. Infection occurs in the tissue and antigen-bearing dendritic cells, as well as free antigens, travel from the site of infection through the afferent lymphatic vessels.
  2. They then drain into lymph nodes where they activate T and B cells.
  3. These lymphocytes undergo selection, differentiation and proliferation before entering the blood via the thoracic duct.
  4. They then travel back to site of infection, where they kill pathogens.

  This cycle occurs over 4-6 days if it is a new antigen that has been introduce to the immune system.
• Describe the structure of a lymph node

  Lymph nodes are kidney-shaped and receive lymph via multiple afferent vessels, and then filtered lymph then leaves via efferent vessels. Nodes typically have an associated artery and vein, which terminates into a high endothelial venue (HEV). The lymph node consists of the capsule, cortex and medulla.
• Describe the cortex in the lymph node

  The cortex of the lymph node is formed of the outer cortex and the inner part is known as the paracortex. The outer layer is rich in B cells arranged into follicles that develops into a germinal centre when challenged with an antigen. Following this, resting B cells and dendritic cells surround the germinal centre to form a mantle zone. The paracortex layer, also rich in T cell that interact with the dendritic cells.
• Describe the medulla in the lymph node

  The medulla is the innermost layer of the lymph node and contains large blood vessels and antibody-secreting plasma cells, B cells and macrophages in medullary cords. The medullary sinuses drains the lymph into the efferent lymphatic vessels.
• Describe the structure of the GALT

  This is a collection of multifollicular structures situated throughout the gastrointestinal tract. The GALT is separated from the intestinal lumen by epithelial cells. It is also connected with the lymph nodes via the lymphatic circulation. The M cell overlies the GALT and exhibit a micro fold structure; they are responsible for transporting pathogens and antigens into the germinal centre of the GALT. The GALT also includes the Peyer’s patches of the small intestine, which are isolated lymphoid follicles found throughout the intestine.

  
• Describe the structure of the spleen

  The spleen is the largest lymphatic organ in the body consisting of two types of tissue called white pulp and red pulp. The white pulp is lymphatic tissue consisting mainly of lymphocytes around arteries and the red pulp consists of venous sinuses filled with blood and cords of lymphatic cells. Blood enters the spleen via the splenic artery and is filtered through the sinuses before leaving via the splenic vein. The white pulp is responsible for the spleen’s immune function, while the red pulp is mostly for removing old blood cells.
• Describe lymphocyte circulation through the systemic lymphoid organs and tissues

  The blood lymphocyte pool either travels to the tissues, the spleen or to the high endothelial venules (HEVs), which are adapted for lymphocyte trafficking. HEVs let lymphocytes leave the blood stream and enter lymph nodes Or they drain back into the blood via the thoracic duct.

  
• Describe the selectin driven stage in lymphocyte adhesion

  1. P-selectin is induced on vessels in response to to certain cytokines, such as leukotriene B4.
  2. E-selection is induced later by TNF and LPS
  3. This allows them to recognise sulphate siayl-Lewis X structures on immune cell and form a weak interaction
  4. This mediates reversible ’rolling’ of immune cells along the vessel walls.
• Describe the firm attachment stage in lymphocyte adhesion

  1. ICAM-1 (intracellular adhesion molecule) and ICAM-2 are induced on vessels by TNF
  2. This allows them to bind to LFA-1 and CR3 on leukocytes and stops rolling as they form a firm attachment
• Describe extravasation of lymphocytes

  1. Leukocytes cross the endothelial wall using LFA-1 and CR3
  2. CD31 interactions also facilitate this

  this is also known diapedesis

  
• Describe examples of chemokines

  • CXCL8 and CCL2 are produced at the site of infection
  • CXCL8: released by macrophages and attracts neutrophils
  • CCL2: produced by epithelial and stromal cells and attracts monocytes
  • CXCL8: produced by macrophage and epithelial cells to mobilise naive T cells