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Updated: Jun 8 2020

Primary Lymphoid Tissue

  • Introduction
    • 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
  • Thymus
    • The thymus is a primary lymphoid organ that is
      • 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
  • Abnormalities
    • 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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