8 TOB Lymph II

• talking about in the lymph node
• medullary cord: loose CT, highly cellular (has reticular fibers, plasma cells, lymphocytes, fibroblasts, neutrophils)

medullary sinus: venous channel running between cords, discontinuous endothelium, traversed by reticular fibers

• splenic nodes and sinuses are similar to lymph node medullary cords & sinuses but:
• 1) have blood cells in BOTH cords & sinuses
• 2) do NOT have reticular fibers transversing the sinuses

• 
• silver stain: recticular fiber gaps represent the spleen sinuses

• lymph is dumped from the afferent lymphatic --> subcapsular sinus --> trabeculum --> medullary sinus then cord then sinus --> efferent lymphatic
• * metastatic cells can also use this pathway to move throughout the body (achilles heel of the lymph system)

• the white invagination = trabeculum 
• can see the lymph node's:
• nodules (follicles) diffuse T-cells, and plasla cells in the medulla CT

• 1) sinuses are lined by a discontinuous endothelium
• 2) reticular fibers traverse ALL sinuses (lymph node not spleen)
• 
• 3) afferent lymphatics outnumber efferent; bottleneck results (take a while for lymph to leave, makes it more likely they'll come in contact with a pathogen)

• High endothelial venules (HEV)
• specialized post-capillary venous swellings characterized by plump endothelial cells (as opposed to the usual thinner endothelial cells found in regular venules)
• lymphocytes circulating in the blood can directly enter a lymph node by crossing through the HEV)
• found in all secondary lymphoid organs (EXCEPT spleen where blood exits through open arterioles and enters the red pulp) including MALTs, tonsils, pharyngeal adenoids, Peyer's patches in the small intestine, & appendix

• Reed-Sternberg cells - present only in present in hodgkin's lymphoma
• 
• characteristic "owl-eye" appearance 
• bilobed nucleus w/ really prominent nucleolus

• virus targets helper T cells and either kills them directly or indirectly as a result of cytotoxic T-cell activity that recognizes infected cells as foreign
• with no helper T-cells, production of activated B lymphocytes drops 
• marked changes evident in germinal centers: do not contain proliferating B- cells & thus produce no “plasmablasts"
• contain only dendritic cells and macrophages

excess proliferation of lymphocytes can lead to lymphomas, solid tumors of the lymphatic system

• 1) Immune response, B & T cells
• 2) Destroys damaged, senescent blood cells
• 3) Sequesters monocytes
• 4) Hematopoiesis (fetal development)
• 5) Storage of blood/platelets
• 6) Recycling of iron

• trabecular artery
• 

trabecular vein has LESS smooth muscle

• the lymphoid component of the spleen organized to maximize interactions between blood borne antigens and cells of the immune system
• consists of both nodular and diffuse lymphoid tissue organized around arteries (PALS)
• arteries surrounded by PALS are known as central arteries
• from these, smaller branches extend to the periphery of the white pulp and terminate in marginal sinuses

• vascular component of the spleen is organized to promote red cell/macrophage interactions
• functions to filter the blood of antigens, microorganisms, and defective or worn-out red blood cells
• terminal, sheathed capillaries empty into the meshwork of red pulp cords (CT) where blood cells must traverse the sinusoidal wall in order to gain entry into the sinuses

traversing the narrow “filtration slits” between adjacent endothelial cells (stave cells) requires flexibility, a characteristic that diminishes with red cell agered cells that cannot traverse the filtration slits will be cleared by macrophages

• loose connective tissue/reticular fibers
• cellular elements: red blood cells, platelets, macrophages, plasma cells lymphocytes

periarterial lymphatic sheath (PALS) surround central arteries and containing a nodule with a germinal center

the Marginal Zone is the interface between the NON-lymphoid red pulp and the LYMPHOID white-pulp of the spleen