Lymphoid System

The Lymphoid (Immune) System

• Responsible for protecting the body against invading microorganisms or foreign bodies
• Distributed throughout the body and consists of aggregations of lymphocytes (B and T lymphocytes) and epithelial cells

Lymphatic Tissues (Organs)

• Classified into:
  • Primary or central lymphoid organs (thymus and bone marrow): where B & T lymphocytes develop
  • Secondary or peripheral lymphoid organs (spleen, lymph nodes, tonsils, appendix, Peyer's patches, and solitary lymphatic nodules): where B & T lymphocytes settle

Thymus

• Flattened bilobed organ lying behind the sternum
• Large in infants, grows until puberty, then diminishes in size
• Consists of:
  • Stroma: capsule, trabeculae, and connective tissue
  • Parenchyma: outer dark cortex and inner pale medulla
• Cortex:
  • Contains thymocytes (T lymphocytes), epithelial reticular cells (APCs), and macrophages
• Medulla:
  • Contains thymocytes (T lymphocytes), epithelial reticular cells (APCs), macrophages, and Hassall's corpuscles
• Functions:
  • Site for development and differentiation of T lymphocytes
  • Production of T lymphocytes
  • Secretion of thymic hormones
  • Formation of Blood-thymic barrier

Spleen

• Single intra-abdominal organ covered by peritoneum
• Consists of:
  • Stroma: capsule, trabeculae, and reticular stroma
  • Parenchyma: white pulp and red pulp
• White pulp:
  • Formed of 4 zones from inside to outside: thymus-dependent zone, germinal center zone, follicular zone, and marginal zone
• Red pulp:
  • Consists of blood sinusoids and splenic cords
• Functions:
  • Hemopoiesis of blood elements during fetal life
  • Production of lymphocytes
  • Destruction of abnormal, damaged, and aged blood cells
  • Blood filtration
  • Storage of blood