Anatomy of the Immune System

Primary Lymphoid Tissue

Bone Marrow

  • Bone marrow is found on the inside of long bones
  • The shaft of long bones has three layers – periosteum, compact bone and then the inner medullary cavity lined by endosteum and containing bone marrow.
  • The bone marrow is the origin of all immune cells.
  • Most cells (including B lymphocytes) develop here.


  • The thymus is an organ with two lobes located behind the sternum within the mediastinum.
  • The thymus gland is where lymphocytes develop into T cells.
  • The outer area is called the cortex, which is where T cells proliferate and start develop into specialised cells
  • The inner area is called the medulla, which is where the mature T cells are found before they enter the blood, and it also contains the waste products of defective T cells


Lymphatic System

  • This is a network of lymphatic vessels, similar to blood vessels
  • There is no built in pump (like the heart)
  • Circulation relies on muscles surrounding the vessels to pump
  • Contain valves to direct the flow of lymph
  • Eventually drains into the circulation via the
    • Thoracic duct (entering between the left subclavian and internal jugular veins) – this is the larger of the two ducts
    • Right lymphatic duct (enteringĀ at right subclavian or internal jugular veins)
  • The lymphatic system contains a fluid calledĀ lymph
    • This is interstitial fluid that have drained from the intersitial space
    • High concentration of lymphocytes and dendritic cells
    • Low protein content
    • No red blood cells or other leukocytes


Lymph Nodes

  • Structures that occur intermittently along the course of lymphatic vessels
    • Afferent vessels drain in to lymph nodes
    • Efferent vessel drains away from lymph nodes
  • Their role is to:
    • Hold lots of cells of the immune system, particularly lymphocytes.
    • Act as filters for pathogens and abnormal cells
  • Shaped a bit like a kidney
  • Surrounded by a capsule
  • Have their own blood supply
  • Have three areas
    • Cortex – contains B cells and follicles
    • Paracortex – contains T cells and dendritic cells
    • Medulla – contains plasma cells, T and B cells and macrophages
    • Hilum – leading to the efferent vessel and where the blood vessels enter
  • Follicles are areas where groups of B cells clump together and wait to be activated
    • Primary follicles contain unstimulated B cells and do not have germinal centres
    • Secondary follicles have germinal centres (that stain a lighter colour) and contain activated B cell that are generating plasma cells and memory B cells



  • The spleen is an organ made up of
    • Red pulp
    • White pulp
  • Located in the left upper quadrant of the abdomen
  • Surrounded by a capsule
  • Red pulp is where old red blood cells are removed (filter through a web of reticular fibres that destroy old RBCs but young ones can squeeze through without getting destroyed).
  • White pulp contains the immune system
    • Immune cells wrap themselves around the arterioles in the spleen forming the white pulp.
    • Immediately around the arteriole is the “peri-arteriolar lymphoid sheath” (PALS)
      • This contains T cells and dendritic cells
    • The marginal zone surrounds the PALS
      • Contains B cells and macrophages
    • At intervals along the arteriole there are follicles
      • Primary follicles contain unstimulated B cells
      • Germinal centres are areas within the primary follicles where B cells have been stimulated and are differentiating into plasma cells and secreting antibodies.


Mucosa-Associated Lymphoid Tissue

  • This is lymphoid tissue associated with the mucosa of the GI, respiratory and urinary tracts.
  • It contains primary follicles and germinal centres similar to the spleen and lymph nodes
  • Much of it is unstructured and appears randomly along the tract
  • Examples of structured MALT tissue is:
    • Tonsils
    • Adenoids
    • Appendix
    • Peyer’s patches (in the ileum of the small intestine)