Lymphoid System in Histology

Lymphoid System Outline

• The lymphoid system consists of primary and secondary lymphoid organs
• Primary lymphoid organs: bone marrow and thymus
• Secondary lymphoid organs: lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT)

Secondary Lymphoid Organs

• Act as filters that trap antigens and expose them to lymphocytes
• Lymph nodes filter lymph, spleen filters blood, and MALT filters antigens that try to cross mucosal surfaces

Lymph Node Components

• Cortex contains lymphoid follicles composed of B cells
• Paracortex contains T cells and high endothelial venules
• Medulla composed of cords containing B cells and macrophages, and sinuses containing reticular fibers, lymph, and circulating cells and antigens
• Stroma composed of reticulin (type III collagen) fibers

Lymph Node Follicles

• Primary follicles consist of small, dark, mature lymphocytes
• Secondary follicles consist of:
  • Germinal center: a lighter-staining central area containing larger lymphocytes
  • Mantle zone: around germinal center; contains small, dark lymphocytes

Afferent and Efferent Lymphatic Vessels

• Afferent lymphatic vessels penetrate the capsule of the lymph node on the convex side and carry lymph with antigen and cells from sites of infection back to the lymph node
• Efferent lymphatic vessels leave through the hilum and carry lymph with lymphocytes from the lymph node and return it to the blood

Functions of the Spleen

• Filters blood (traps antigens)
• Provides a place for lymphocytes to encounter antigens and become activated
• Removes old red cells and platelets

Structure of the Spleen

• Parenchyma of spleen is called splenic pulp and consists of white pulp and red pulp
• No cortex or medulla are present
• Stroma consists of capsule, trabeculae, and reticular connective tissue
• Hilum contains nerves, arteries, and veins

White Pulp of the Spleen

• Consists of:
  • Periarteriolar lymphatic sheaths (PALS): tubule-shaped collections of T cells around arterioles
  • Splenic follicles (nodules): similar to secondary follicles in lymph nodes, with an additional marginal zone composed of B cells

Red Pulp of the Spleen

• Consists of:
  • Splenic sinuses (sinusoids): leaky vessels lined by specialized endothelial cells and surrounded by rings of reticular fibers
  • Splenic cords (cords of Billroth): contain lots of macrophages that eat old or damaged red cells

Mucosa-Associated Lymphoid Tissue (MALT)

• Unencapsulated lymphoid tissue in mucosa and submucosa of digestive, respiratory, reproductive, and urinary systems
• Catches antigens as they try to cross the mucosa and get into the body
• Found in lung and other systems

This quiz covers the primary and secondary lymphoid organs, their functions and roles in fighting infections. Understand the lymph nodes, spleen, and MALT.