Lymphoid System I Flashcards
What are the primary lymphoid organs?
Thymus and bone marrow
What are the secondary lymphoid organs?
Lymph nodes, spleen, tonsils, epithelial/mucosa-associated lymphoid tissue.
What are the secondary lymphoid organs designed to do?
Designed to maximize exposure of foreign antigens to the immunocompetent cells.
What cells are responsible for cell-mediated immunity?
T-lymphocytes
What cells are responsible for humoral immunity?
B-lymphocytes
What cells differentiate to become plasma cells for Ig synthesis?
B-cells
What are antigen presenting cells (APCs)?
Phagocytic cells that take up and process antigens, then present the antigens to T-lymphocytes.
Where are B cells made?
Bone marrow
What does “bursal equivalent” refer to?
The pockets in the bone marrow that make B cells (derived from bursa of Fabricius in birds)
Where are T-lymphocytes made?
Bone marrow. They are educated in the thymus
What percentage of T cells make it through the selection process?
Only 5%
What causes the “starry sky” appearance of the thymus?
Macrophages that clear out all the apoptosed T cells create spaces where nuclei of the T cells are less dense.
What do the epithelial reticular cells (ERCs) do? Do they have reticular fibers?
They provide mechanical support for developing cells in the thymus. They do NOT have reticular fibers.
What is the CT septa?
The thymus lobes’ equivalent to the perimysium
What does the Thymic cortex do?
Contains ERCs for mechanical support, produces thymic hormones thymosin and thymopoietin, promotes T cell maturation, education of thymocytes (self vs non-self), and formation of the blood-thymus barrier.
What does the Thymic Medulla do? How does it stain? What structure is unique to the Thymic medulla?
Stains lighter due to fewer lymphocytes and lots of macrophages. Also has Hassall corpuscles (ONLY found in medulla). The cortico-medullary boundary has specialized post-capillary venules that have thick endothelium where the T cells exit the thymus to enter the circulation.
What is a Hassall corpuscle? What do they do?
Concentric layers of epithelial cells that continue to grow over time. They make thymic stromal lymphopoietin (TSLP) that helps regulatory T cells differentiate.
What does involution and adipose tissue in the thymus indicate?
That the thymus is of a post-pubescent person. During adolescence, this happens once the seeding of secondary lymph organs is complete. Hassall corpuscles will continue to grow after this, however
Which secondary lymph organs are encapsulated (covered by CT)?
Lymph nodes and spleen.
What is MALT? Are they encapsulated?
Mucosa-associated lymphoid tissues - tonsils, Peyer patches, veriform appendix. Unencapsulated. Contains both nodular and diffuse lymphoid tissue
What is nodular lymphoid tissue?
Compact, well defined collections of lymphoid tissue, reticular cells and associated fibers that support the cells.
The presence of both nodular and diffuse lymphoid tissue is diagnostic of what?
A secondary lymphoid tissue.
What is diffuse lymphoid tissue?
Found between nodules and in the CT underlying most epithelia, supported by reticular fibers and contains mostly T-lymphocytes and macrophages.
What is a germinal center?
Found ONLY IN NODULAR TISSUE, contains many enlarged, proliferating lymphocytes and stains lighter. B cell activation happens here
Do B cells activate in diffuse tissue?
NO, only T cells
Do tonsils have nodular and diffuse lymphoid tissue? What is diagnostic of tonsils?
Yes. Tonsils have crypts (invaginations) to increase surface area for max interaction between antigens and lymphocytes.
Name the three major tonsils.
- Palatine (paired) - found in the lateral walls of the fauces
- Pharyngeal (single) - found on the posterior wall of the nasopharynx
- Lingual (multiple) - located on posterior tongue
Describe the epithelial cell type on palatine tonsils.
Stratified squamous epithelium
Describe the epithelial cell type on pharyngeal tonsils.
Pseudostratified ciliated columnar
Describe the epithelial cell type on lingual tonsils.
Stratified squamous epithelium
What are Peyer Patches and where are they found?
They are structures of 30-50 nodules with intervening diffuse tissue found in intestinal mucosa and/or submucosa in the ileum and along the abmesenteric wall of the ileum.
What are microfold cells (M cells)?
Found in appendix and in the epithelium of Peyer patch nodules, they have deep invaginations that bring immune cells close to the gut lumen. They are the basis of oral vaccination. Seen best under EM.
What is unique about Peyer patch basement membrane?
It is discontinuous to allow for rapid movement of immune cells in and out of epithelium
What type of lymph tissue is found in the appendix?
Both nodular and diffuse
What is the function of the appendix?
- Reserve of gut flora
- Endocrine role
- Immune surveillance
What is diagnostic of the spleen?
The random distribution of the white pulp (lymphatic tissue) throughout the organ and surrounded a “sea” of red pulp (splenic blood channels).
Is the thymus covered by an epithelium?
No, it is encapsulated so there is no epithelium.
What are ERCs and where are they found? What do they do?
Epithelial reticular cells. Found in thymus, provide mechanical support with cytokeratin, connected to each other with desmosomes.
What is the direction of flow of the T cells through the thymic High Endothelial Venules?
The T cells leave through thymic HEVs to enter the circulation, and go on to secondary lymphoid organs.
