Thymus & Lymph Nodes Flashcards
What does lymph consist of? What do the lymphatic vessels do with these contents?
Extracellular fluid and debris - they drain them to regional lymph node for immune surveillance
Describe the structure of the thymus.
Composed of a medulla (inner, lighter region) and a cortex (outer, denser region) that are located in mediastinum and are divided into irregular rectangular lobes by septae of connective tissue
Which part of thymus is rich in T lymphocytes?
Cortex
Which part of thymus is rich in epithelial reticular cells/epitheliocytes?
Medulla
What type of intermediate fiber is found in epithelial reticular cells?
Keratin
How do epithelial reticular cells communicate with each other?
Desmosomes
Are epithelial reticular cells stromal?
No - they do not have extracellular reticular fibers
What are two functions of epithelial reticular cells?
Form lattice network that engulfs the developing T cells, aids in formation of thymic blood barrier
What are Hassall’s Corpuscles?
Flattened, concentrically arranged epithelial reticular cells that are degenerating.
What are the 5 layers of the blood-thymus barrier?
Endothelial cell of blood vessel, basal lamina of endothelial cell (thick), layer of connective tissue (fibroblasts and ECM), basal lamina of epithelial reticular cell, epithelial reticular cell itself
What part of the thymus is rich in T cells?
Cortex
Where do T lymphocytes migrate to the cortex from?
Bone marrow
What part of the thymus do T lymphocytes enter first?
Subcapsular cortex
In what part of the thymus does positive selection process for MHC marker take place?
Cortex
If cells do not positively select for MHC, what happens to them?
Apoptosis or get eaten by macrophages
If T lymphocytes recognize MHC and pass first selection process, where do they migrate next?
Medulla
What selection process happens in the medulla? Which cells present the antigen in question?
Negative selection for recognition of self antigens - dendritic cells present the self Ags
What happens to mature T cells (the 2% who pass both selection processes)?
They re-enter vascular circulation through leaky blood/thymus barrier in medulla.
What makes up secondary lymphoid tissue?
Spleen and lymph nodes, and tonsils (diffuse lymphoid tissue)
Describe the general structure of lymph nodes and general function.
Kidney bean-shaped, encapsulated filters that filter regional lymph fluid to neck, axilla, groin, para-aortic regions.
What are two main regions of lymph node?
Cortex (outer region) and medulla (inner region)
What types of structural cell exists in lymph node? What type of fiber?
Reticular cells (fibroblasts) and reticular fibers (collagen Type III)
Where do afferent lymphatics enter the lymph node?
Subcapsular sinus in outer convex surface of cortex
Where does the single efferent lymphatic vessel exit the LN?
Hilus on inner concave surface of cortex
What does lymph percolate through in the lymph node?
Septae of connective tissue that is composed of reticular cells (fibroblast-like) and reticular fibers
Where do lymphocytes enter the lymph node?
They are circulating in blood and enter through high endothelial venules (HEVs) located in paracortex region
What types of immune cells exist in lymph nodes?
B cells, T cells, plasma cells, macrophages
Where in lymph node are B cells found? How are they grouped?
Cortex - they are grouped into primary and secondary nodules
What is a germinal center?
Clear area that is site of B cell activation and proliferation on secondary B cell nodules
Where do B cells go as they mature? And what do they become?
Become plasma cells that move more into deep cortex (paracortex) and medulla region
What immune cells are found in medulla?
Plasma cells and macrophages
Where do T cells in lymph nodes reside?
Internodal space
Explain how T lymphocytes are activated for clonal selection.
Antigen on MHC II of APC binds to CD4 of helper T cell and its TCR. Helper T cell releases IL-2, to stimulate helper T cells. Activated helper T cells then proliferate and differentiate to form clones of both activated and memory helper T cells.
Explain two ways in which cytotoxic T cells are activated.
1) CD8 on cytotoxic T cell binds with MHC class I molecule of various cells - the TCR interacts w/ Ag on MHC I molecule.
2) IL-2 released from activated helper T cells stimulates cytotoxic T lymphocyte.
Explain two ways B cells are activated.
1) Free Ag binds to BCR. B cell engulfs BCR and presents Ag to activated helper T cell using MCH II.
2) IL-4 released from activated helper T cells stimulates B cells so they proliferate and differentiate to form clone of plasma cells that will release Abs and memory B cells.
