Lymphoid System I & II Flashcards
Primary Lymphoid organs - Name them and describe function
- Thymus
- Bone Marrow
provide a protected environment for the development of immunocompetent T and B lymphocytes (cells that have receptors for a specific antigen [not a self antigen] and that have not yet encountered that specific antigen)
Secondary Lymphoid organs- name them and describe function
Unencapsulated, MALT (distinguishable by their epithelial covering):
- Tonsils (lingual, palatine, pharyngeal)
- Peyers Patch
- Appendix
Encapsulated:
- Lymph nodes
- Spleen
- masses of lymphocytes and associated cells required to mount an immune response
- provide an environment that promotes immune cell-antigen interaction
- may exist as discrete organs covered by either an epithelium or
connective capsule - or - may exist as isolated masses of cells within various organs that are in close proximity to the outside world
- ALL HAVE NODULAR/DIFFUSE LYMPHOID TISSUE
Primary and Secondary Lymph Organs
Yellow are primary, blue are secondary
Thymus
Primary lymphoid organ
Has a “starry sky” appearance due to the abundance of macrophages
Has a continuous, branched medulla and septa
Thymic epithelial cells form a cytoreticulum that becomes infiltrated by T-cell precursors (thymocytes)
Cytoreticulum
Area formed by thymic epithelial cells that becomes infiltrated by T-cell precursors (thymocytes)
Epithelioreticular cells
cytokeratin positive cells that support thymocytes(the thymus has no supporting reticular fibers)
- surround capillaries & contribute to blood thymus barrier (thus preventing foreign antigens from entering the thymus and thus being considered “self”)
- a subset form Hassall’s corpuscles
- Not visible without special stain (keratin immunohistochemistry, staining a dull gold)
Hassall’s corpuscles
whorls of epithelioreticular cells residing in thymic medulla
-poorly understood but appear to regulate T-cell (TR) development
Nodular Lymphoid Tissue
contains compact, well-circumscribed collections of lymphoid tissue supported by reticular cells and their associated fibers
Germinal Center
more lightly stains central region of lymphoid nodules which contains many enlarged, proliferating lymphoblasts
Mucosa Associated Lymphoid Tissue (MALT)
Unencapsulated tissues, covered by an epithelium, include a variety of organs collectively known as the Mucosa Associated Lymphoid Tissues (MALT)
Palantine Tonsils
Have distinguishing SSNKE and crypts
Pharyngeal Tonsils
Have pseudostratified ciliated columnar epithelium with goblet cells
Can contain wandering lymphocytes
Peyer’s Patch
structures consisting of 30 to 50 nodules with intervening diffuse lymphoid tissue found in the intestinal mucosa and/or submucosa, typically found in ileum
Have simple columnar epithelium, contains microfold cells (M cells)
M cell
Microfold cells
On the surface of Peyer’s Patch are specialized cells with large invaginations that forms a pocket in which immune cells reside in order to sample antigens
Basis for oral vaccination
Appendix
attached to the bottom of the ascending colon
Functions:
- Immune surveillance (M-cells are abundant)
- Vestigal – former enlarged cecum
- Endocrine organ - endocrine cells appear by 11th week in fetus
- Reserve of gut flora
- following loss due to diarrhea, toxic compounds, radiation, use of broad-spectrum antibiotics, etc. (Circumvents the need for probiotics or fecal transfers)
- Surgical substitute for diseased ureters, sphincter in reconstructive bladder surgeries
Lymph node
provide a line of defense against antigens that penetrate the epithelium and MALT and gain access to the underlying connective tissues
- range from 1mm – several cm
- concentrated in neck, axilla & groin
- filter lymph and expose antigens to immune cells
Contains outer cortex, paracortex, and medullary sinuses and cords
Nodules are in the outer cortex
Medullary cord
within lymph node
loose CT, highly cellular
Medullary Sinus
within lymph node
venous channel running between cords, discontinuous endothelium, traversed by reticular fibers
Splenic Cords & Sinuses
similar to lymph node medullary cords & sinuses but:
- have blood cells in both cords & sinuses
- sinuses are not traversed by reticular fibers
ID Items in Red
Three mechanisms that assure interactions between lymphocytes and pathogens (in lymph node)
1) Medullary sinuses are lined by a discontinuous endothelium
2) Reticular fibers traverse all sinuses (lymph moves more slowly through RFs)
3) Affarent lymphatics (many entering) outnumber efferent lymphatics (one exiting) resulting in a bottleneck
High endothelial venule (HEV)
Found in the inner cortex of lymph nodes; they permit circulating lymphocytes in the blood to enter the node
They are the main route of lymphocyte entry; few cells are brought in through the afferent lymph
has more cuboidal endothelial cells
Selectins allow lymphocytes to home in on areas in the HEV, then attach and crawl through the walls and enter the parenchyme
Lymphadenopathy
Following viral infection, HEVs become activated:
- express greater numbers of selectins such that their surface becomes especially sticky to passing lymphocytes (especially cytotoxic T cells)
- more and more lymphocytes enter the nodes causing the paracortical region to swell (paracortical reaction)
- Enlargement of nodes can become pronounced; sometimes referred to as “swollen
glands”.
HIV Infection Mechanism
The HIV virus targets helper T-cells, either killing them directly or indirectly as a result of cytotoxic T-cell activity that recognizes infected cells as foreign
(HIV infiltrates T cells via CD4 receptors, where it replicates)
When help T-cell population depleted, production of activated B-lymphocytes is severely curtailed
Germinal centers will contain only dendritic cells and macrophages; no proliferating B cells– hence, production of anti-body producing plasma cells cannot occur
Spleen
Functions:
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
Covered by a squamous epithelium (mesothelium) with an underlying thick connective tissue capsule
No cortex or medulla arrangement in the spleen but instead there are scattered regions of lymphoid cells (white pulp) (stains darker in H&E) among the more vascularized regions called red pulp
Also contains trabecular arteries and veins within trabecular beams
Trabecular artery vs Trabecular vein
Found in spleen
Red pulp cords
Found in spleen
- loose connective tissue/reticular fibers
- cellular elements:
red blood cells, platelets, macrophages, plasma cells
lymphocytes
Red pulp sinuses
Found in spleen
- venous channels lined by elongated, discontinuous endothelial cells
- discontinuous basal lamina
Marginal Zone
In the spleen; region between the red and white pulp and not only contains sinuses possessing reticular fibers, but also many macrophages
marginal zone receives blood from the central arteries and the network of reticular fibers slows the blood flow and thus promotes interaction of blood antigens to the macrophages as well as to lymphocytes and other immune cells of the PALS. The trapping ability of this zone can be demonstrated by draining the blood from the spleen. The blood fails to drain from the marginal zone region (or at least is drained at a much lower rate than in other areas).
ID the arrows
Hassal’s Corpsucles
Found in medulla of thymus
ID the tissue
Lymphoid nodules
Found in secondary lymph organs
ID the organ/describe
Lymph node
ID the organ/describe
Thymus
ID the organ/describe
Palatine tonsil
ID the organ/describe
Peyer’s patch
ID the organ/describe regions
Spleen
Describe
Contains the Peyer’s Patch
ID the organ/describe
Pharayngeal tonsil
Has ciliated pseudostratified columnar epithelium with goblet cells and wandering lymphocytes
Find the location of M cells
ID the structure
HEV
Found in the inner cortex of lymph nodes; they permit circulating lymphocytes in the blood to enter the node
The central region of this image depicts which one of the following?
Thymus medulla
Hassal’s Corpuscles tells us this is the thymus medulla
The white circle is surrounding T-lymphocytes
Mucosa
Epithelial lining of an internal organ plus the lamina propria and the smooth muscle layer just deep to it
ID the organ/describe
Palatine tonsil
contains SSNKE
contains crypts, the dark cap of the nodule always faces the crypt
Humoral Immunity
involes the production of antibodies by plasma cells
ID organ and labels
Lymph node
1) efferent lymphatic
2) afferent lymphatic
3) trabecula
4) subcapsular sinus
5) trabecular sinus
