10 Lymphoid_2

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  1. Appendix Functions
    • involved in immune surveillance (because it has so many M-cells & lymphocytes)
    • potentially a vestigal structure from a formerly enlarged cecum
    • can be an endocrine organ; see endocrine cells by 11th week in fetus
    • is a reserve of gut flora; can help repopulate gut after loss following diarrhea, toxic compounds, radiation, broad-spectrum antibiotics, etc. (don’t need fecal transfer)
    • can be a surgical substitute for diseased ureters, sphincter, in reconstructive bladder surgeries
  2. Encapsulated Secondary Lymphoid Organs
    • Lymph Nodes
    • Spleen
    • these 2 have a robust CT capsule covering them
    • once you have immunocompetent B & T cells, they populate 2ndary lymphoid organs
  3. Lymph
    • the plasma component of the blood
    • it escapes from capillaries + post-capillary venules & enters CT
    • from CT it’s picked up by blind ended lymphatics & through them will work its way through the body where it will encounter lymph nodes
  4. Lymph Nodes
    • Image Upload 1
    • found throughout the body but are concentrated in neck, axilla & groin
    • range from 1mm → several cm
    • filter lymph & expose antigens to immune cells
    • Image Upload 2
    • can see CT covering
    • lots of lymphocytes (see nodular + diffuse areas → know it’s a secondary lymphoid organ)
    • nodules tend to be in the outer cortex near the CT capsule, diffuse T cells tends to be in the inner (Para) cortex
    • Medulla contains medullary cords (dark areas) & sinuses (spaces in between)
  5. Lymph Node Medullary Cords
    • a loose CT because it contains reticular fibers
    • is very cellular as well: contains plasma cells, lymphocytes, fibroblasts, neutrophils
    • Image Upload 3
  6. Lymph Node Medullary Sinuses
    • venous channels running between the cords, dividing them up
    • sinuses are lined by a discontinuous endothelium, so cells can move between the cord & sinus
    • reticular fibers span the sinusoids
  7. Splenic Cords & Sinuses
    similar to lymph node medullary cords & sinuses, except:

    1. blood cells are found in BOTH cords & sinuses

    2. reticular fibers do NOT span the sinuses
  8. Lymph Flow
    • lymph passes into lymph nodes through Afferent Lymphatics (have valves)
    • these pierce CT capsule
    • once inside, the lymph gets dumped into a space (subcapsular sinus) just deep to the CT capsule
    • subcapsular sinuses are continuous with trabecular sinuses that follow CT trabeculae that go into the medulla
    • trabecular sinuses are continuous with medullary sinuses (sinuses in the medulla)
    • these all coalesce into a single efferent lymphatic
    • this efferent lymphatic may become an afferent lymphatic for another lymph node (the lymph has many opportunities to undergo filtration)
  9. Where will a plasma cell never be found?
    • germinal center of a secondary lymphoid organ
    • before terminally differentiating, B cells migrate from nodules to nearbly CT & then differentiate into plasma cells
    • the nearest CT = medullary cord; will find lots of plasma cells there
    • Image Upload 4
  10. What 3 mechanisms ensure there are interactions between incoming lymph containing antigens & immune cells in the lymph nodes?
    • 1. sinuses are lined by a discontinuous endothelium - cells can easily pass back & forth between
    • 2. reticular fibers traverse all (lymph node) sinuses - form a dense network that slows lymphocyte & lymph movement, allowing for increased likelihood of interaction
    • Image Upload 5
    • 3. bottleneck: afferent lymphatics outnumber efferent - much more lymph is coming in than can escape, tends to slow lymph flow down; plenty of time for it to interact with the lymphocytes
  11. How do most lymphocytes present in a lymph node reach the node?
    via the blood stream
  12. HEVs (High Endothelial Venules)
    • found in the post-capillary region of the circulation system in the cortex of lymph nodes
    • little ‘swellings’
    • endothelial cells in these areas are much more cuboidal than elsewhere (most endothelial cells in body are squamous)
    • this is the ONLY area where lymphocytes will escape the blood & enter various organs - cells stick & crawl through
  13. HEV Migration
    • Image Upload 6
    • pale nuclei: endothelial cells
    • dark nuclei: lymphocytes
    • lymphocytes have stuck to the wall of endothelial cells & are crawling through → enter the lymph node proper
    • normally there’s a constant # of lymphocytes in a lymphatic: # of lymphocytes that enter a node via HEV migration = the same as those that leave via efferent lymphatic
  14. What happens to a lymph node during a viral infection?
    • Lymphadenopathy - growth/expansion of a lymph node
    • viral infection activates cuboidal endothelial cells of the HEV → increased levels of selectin molecules are expressed on their surface
    • endothelium becomes STICKY for lymphocytes; more stick & crawl through to node
    • many more lymphocytes enter lymph node that can leave via efferent lymphatic
  15. When are HEVs located?
    • the Paracortex (inner)
    • during illness, because there’s an influx of lymphocytes in this area, it EXPANDS
    • entire lymph node gets larger - called the Paracortical Reaction
    • Image Upload 7
  16. HIV Infection
    • virus takes advantage of CD4 molecule on surface of helper T cells to bind to T cell surface
    • B cell needs to bind to a helper T cell to undergo differentiation into Plasma cell
    • with HIV, you lose helper T cell population (via cytotoxic T cells that recognize the infection) → person can’t make antibodies
  17. Spleen Functions
    • 1. Immune response, B & T cells
    • 2. Destroys damaged, senescent blood cells
    • 3. Sequesters monocytes (there are lots of macrophages in the spleen, they come from monocytes in the blood)
    • 4. Hematopoiesis (fetal development)
    • 5. Storage of blood/platelets (~30%, store less blood)
    • 6. Recycling of iron (because we’re clearing RBCs)
  18. Red Pulp
    • area where there’s mostly blood, very few lymphocytes
    • Image Upload 8
    • notice there’s NO cortex + medulla arrangement - just have an outer cortex then inside a random scattering of red & white pulp
  19. White Pulp
    • areas where there are masses of lymphocytes
    • are so concentrated they exclude the blood (that’s why they’re white)
  20. What typically follows the CT capsule trabeculae as they insert into the spleen?
    • splenic artery vessels
    • trabecular veins also follow the CT capsule invaginations of the of spleen
    • Image Upload 9
  21. Red Pulp Cords & Sinuses
    • red pulp cords have lots of reticular fibers (is loose connective tissue) + cells (RBCs, platelets, macrophages, plasma cells, lymphocytes)
    • Image Upload 10
    • red pulp sinuses LACK reticular fibers
    • Image Upload 11
  22. Central Arteries
    • what splenic artery vessels are called/turn into when they lose their CT capsule from the cortex
    • are instead now surrounded by LYMPHOCYTE sheath called the PALS: periarterial lymphatic sheath
    • beneath the PALS are the red/white pulp (cord + sinus) areas more central to the spleen
  23. Marginal Zone
    • border between the PALS & red/white pulp (cord + sinus) areas where central artery branches end
    • it’s AT the marginal zone where any foreign antigens present in the blood will first get access to splenic lymphocytes
  24. What is present & important in the Marginal Zone?
    • Reticular Fibers - they slow the blood flow down so any foreign antigens present can mix with lymphocytes in the PALS
    • can initiate immune responses as needed
  25. How can you demonstrate the Marginal Zone’s ability to slow the blood down?
    • draining blood from the spleen
    • will see red pulp sinuses are drained, but red HALOS around white pulp areas
    • represents marginal zones where blood flow is slowed down
  26. Open Circulation
    • central artery branches terminate in red pulp areas
    • RBCs aren’t directly entering a sinusoid, but instead are entering the red pulp area
    • the only way the blood can get back to the body is by traversing the wall of the sinusoid
  27. How do RBCs escape the red pulp?
    • by traversing the wall of the sinusoid, which is made up of Stave Cells
    • endothelial cells have gaps in between them, called filtration slits (are quite narrow)
    • have a discontinuous basal lamina that surrounds the Stave endothelial cells
Card Set:
10 Lymphoid_2
2016-10-13 00:07:51
MedFoundationsI Histology Exam2
Histology Exam 2
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