Lymphatic System: Lymph Nodes, Bone Marrow, Thymus, Spleen, GALT Flashcards
Why is the Lymphatic System required? Hint: What does it do to fluid that leaks out of capillaries?
Need for this type of system comes from major problems posed by blood system.
Heart > arteries > arterioles > capillaries < venules < veins <<< heart.
Fluid leaks out at capillaries - must get back in. Too much pressure to re-enter blood stream. (Diffusion not possible.)
Lymphoid system picks up leaked fluid from blood (wastes, CO2, etc.) and returns it to the heart. Wherever there are capillaries, there are lymphatics.
Primary function of lymphatic system is to make blood circulation closed.
What happens to the blood fluid/lymph that leaks out?
It gets picked up in local capillary sinuses. These coalesce into lymph vessels of increasing size. They form into ducts (eg. thoracic). It finally returns to blood in neck, completing closure.
What is lymph?
**All fluid outside cells that is not in bloodstream is Lymph.
What is the second function of the Lymphatic System?
It drains every cubic mm of body.
Lymph nodes screen lymphatics for invaders.
It is packed with white cells to mount response (lymphocytes, macrophages, DCs, but NO granulocytes).
What is the architecture of Lymph Nodes?
Lymph enters via afferent vessels, exits via efferent vessels.
Material percolates down through various layers of node.
What are Reticular cells?
They are part of the Lymphatic System.
Reticular cells: fibrous cells forming a sort of skeleton. They are like dendritic cells, but are phagocytic. They are found throughout the node. They help trap Antigen passing through node.
What is the Cortex?
It is part of the Lymph Node, which is part of the Lymphatic System.
It contains mostly virgin B cells (IgM, IgD).
Local concentrations are called follicles (primary and secondary).
It has follicular dendritic cells to trap Antigen.
Immunologically experienced animals have Germinal Centers. If see Antigen, proliferate. Proliferation leads to Germinal Centers.
It has Memory cells in mantle of secondary follicle.
What is the Paracortex?
It is part of the Lymph Node, which is part of the Lymphatic System.
It contains mostly T cells.
It moves up to Cortex to help B cells.
May also _exit node to patrol blood. _
What is the Medulla?
It is part of the Lymph Node, which is part of the Lymphatic System.
Fewer cells.
Reticular fibers condense into “medullary chords.”
Contents empty into efferent lymph vessel.
Points of lymphatic egress from LNs.
B cells differentiate here to become Plasma cells. Plasma cells make Antibodies, generate memory. They soon spill over, and go to Bone Marrow or Germinal Center in other nodes. It is a good way to spread memory around the body.
How it works:
Antigen enters node through afferent lymph vessel.
Enters the cortical sinus, and percolates through node. Reticular cells and DCs in LN help trap Antigen.
Macrophages within node trap and present Antigen.
If foreign, system reacts.
Either or both virgin B cells or memory B cells may react.
What is the Bone Marrow?
Myeloid tissue. Makes white cells, core of long bones. 2-3 Liters in adult.
3 Major functions:
1. Produce/support stem cells
2. B cell maturation
3. Spill-over Antibody production in memory responses.
Includes Stem Cells:
All hematopoietic cells are derived from stem cells.
Stem cells are undifferentiated.
Stem cells divide asymmetrically. One daughter cell will differentiate, the other will remain in the Bone Marrow.
Sensitive to radiation, chemotherapy (treatments for cancer.)
This sensitivity is the main barrier to eradication of tumors.
What is the architecture of the Bone Marrow?
Has microenvironments for development of each cell type.
Provides specific factors which guide differentiation.
Signals from periphery tell Bone Marrow what cells to make.
Where do B cells complete their differentiation?
In the Bone Marrow.
Virgin B cells:
In Bone Marrow - are IgM+, IgDweak (immature Virgin B cell).
In LN - become IgM+IgD+ (mature Virgin B cell).
Bone Marrow also contains memory B cells, like Germinal Center in LN. Spillover from LN, spleen.
What is the Thymus?
Thought to be Antigen-free.
Second master organ.
Referred to as “central” lymphoid tissue with Bone Marrow (vs. peripheral = LN, spleen)
2 Major functions:
1. Differentiation of T cells: Pre-thymocytes arrie via bloood, move into subcapsular space. Differentiative events begin immediately. Differentiation markers expressed: CD3, TCR; CD4, CD8. Get rearrangement of TCR gene fragments, like B cells. Begin migrating towards medulla.
*CD4 or CD8 fate decided here.
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Recognition of self vs. non-self: Selection of TCR (mid-cortex), must recognize self but not too strongly (cortical-medullary interface). By the time they reach medulla, they are essentially fully mature. Exit to LN, spleen.
*Thymic atrophy - T cells not always replenished.
**Thymus differs from LN, even BM, in that no immune reactions take place there, except in certain diseases.
What is the Spleen?
Like a big LN; filters blood instead of lymph. Immunologically, has the same functions.
Also has other functions:
Red Pulp: Removal of RBCs, recover iron.
White Pulp:
Has no afferent lymphatics; WBC can only arrive via blood.
Move out of blood into organized regions, “white pulp”.
Definite locus of immune reactivity.
WBC can leave via lymph (all tissues drained by lymph).
What is the Gut-Associated Lymphoid Tissue (GALT)?
Largest lymphoid tissue.
Contains IgA+ B cells & γδ T cells present. Also contains Peyer’s patches.
M cells: phagocytose and transport Ag across epithelium - activates B cells in underlying lymphoid follicles. Activated B cells differentiate into IgA-producing Plasma cells, which migrate along the submucosa & secrete IgA. The outer mucosal epithelial layer contains intraepithelial lymphocytes, which often are CD8+ T cells expressing γδ TCRs w/ limited receptor diversity for Antigen.
