• There are two primary lymphoid organs in humans including the
    • bone marrow
      • the site of development for B-cells
    • thymus
      • the site of development for T-cells
  • They serve as the site of adaptive immune cell maturation steps such as
    • replication of an
      • immature progenitor cell population
    • recombination of VDJ regions to
      • generate adaptive immune cell receptors
    • positive selection to remove
      • cells with defective surface receptors
    • negative selection to remove
      • cells that are reactive against self-antigens
  • Abnormalities in primary lymphoid tissue can result in
    • immunocompromise due to
      • failure of adaptive immune cell development
    • autoimmunity due to
      • failure of central tolerance mechanisms (negative selection)
Bone Marrow
  • Bone marrow is the site of pluripotent hematopoietic stem cells that can
    • differentiate into many different lineages of which the two relevant to immunity are
      • the lymphoid lineage, which
        • is stimulated by the cytokine IL-7
        • includes members of both the adaptive and innate immune response
          • B-lymphocytes (adaptive)
          • T-lymphocytes (adaptive)
          • NK cells (innate)
      • the myeloid lineage which
        • is stimulated by GM-CSF and IL-3
        • includes members of the innate immune response
          • monocyte/macrophage
          • eosinophils
          • mast cells
          • basophils
          • platelets
          • erythrocytes
          • dendritic cells
  • Bone marrow also serves as the site of B-cell maturation where
    • the B-cell receptor is recombined
    • immature B-cells undergo both positive and negative selection
  • The thymus is a primary lymphoid organ that is This photomicrograph shows a hassall corpuscle, which is a characteristic feature of the inner medulla of the thymus and is composed of epithelial reticular cells.
    • located in the anterosuperior mediastinum
    • derived from the third pharyngeal pouch
    • bilobed in structure with a dense cortex and a pale medulla
    • the site of Hassall corpusles containing epithelial reticular cells
  • The thymus serves as the site of T-cell maturation where
    • the T-cell receptor is recombined
    • immature T-cells undergo both positive and negative selection
  • The thymus is composed of several regions including
    • an outer cortex region with
      • immature T-cells
      • a network of epithelial cells, dendritic cells, and macrophages
    • a corticomedullary region that serves as the site for
      • positive selection of T-cell receptor development
      • negative selection against autoimmune T-cells
    • an inner medullary region containing
      • mature T-cells
      • epithelial reticular cells
      • Hassall's corpuscles
  • Abnormal development of primary lymphoid organs can lead to
    • immunodeficiency syndromes such as 
      • severe combined immunodeficiency (SCID)
      • thymic aplasia (DiGeorge syndrome) which
        • is caused by 22q11 deletion leading to
          • failure to develop 3rd and 4th pharyngeal pouches
          • ↓ PTH due to parathyroid aplasia causing 
            • tetany due to hypocalcemia
          • T cell deficiency due to thymic aplasia leading to
            • recurrent viral/fungal infections
            • absent thymic shadow on CXR
          • congenital heart and great vessel defects
    • autoimmunity syndromes such as
      • myasthenia gravis associated with
        • development of a thymoma
      • generalized autoimmunity due to
        • failure of central tolerance



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