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Primary lymphoid organs

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Topic updated on 04/20/17 6:15pm

Overview
  • There are two primary lymphoid organs
    • bone marrow
    • thymus
  • Both are sites of immune cell maturation from immature progenitor cell population
Bone marrow
  • Site of pluripotent hematopoietic stem cells
    • differentiates into 2 lineages
      • lymphoid
        • stimulated by IL-7
        • considered adaptive branch
        • immune cells it gives rise to
          • B-lymphocytes
          • T-lymphocytes
          • NK cells
            • considered part of innate immune system
      • myeloid
        • stimulated by
          • GM-CSF (granulocyte/monocyte-colony stimulating factor)
          • IL-3
            • secreted by T-cells
        • considered innate branch
        • immune cells it gives rise to
          • monocyte/macrophage
          • eosinophils
          • mast cells
          • basophils
          • platelets
          • erythrocytes
          • dendritic cells
  • Site of maturation of B-cells
Thymus
  • Immature T-lymphocyte progenitor cells leave bone marrow and travel to thymus
  • Structure
    • bi-lobed
    • in anterior mediastinum
    • outer cortex region
      • immature T-cells
      • network of epithelial cells, dendritic cells, macrophages
    • corticomedullary region
      • site of + and - selection
    • inner medulla region
      • maturing T-cells
      • contains epithelial reticular cells + Hassall's corpuscles
      • histologically pale
  • Thymic aplasia (DiGeorge syndrome)
    • 22q11 deletion
    • failure to develop 3rd and 4th pharyngeal pouches
    • presentation
      • ↓ PTH due to parathyroid aplasia
        • tetany due to hypocalcemia
      • T cell deficiency due to thymic aplasia
        • recurrent viral/fungal infections
          • humoral-mediated immunity still functional and can fight bacterial infections
      • congenital heart and great vessel defects
        • also formed from pharyngeal pouches
      • absent thymic shadow on CXR

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