Lymphoid Tissue Flashcards
What are the two functional classification of lymphoid tissue, and what characterizes them?
Primary: Located in the thymus and RBM. Produces lymphocytes needed to recognize Ags
Secondary: Diffuse lymphoid tissue where lymphocytes are activated in response to Ags
Where is diffuse lymphoid tissue located?
Lamina propria of GI, genitourinary tract, and respiratory passages.
Define lymphatic nodules
Discrete concentration of lymphocytes
How can you distinguish a primary nodule on a histological image?
The lack of a germinal center
What is the difference between a primary and secondary lymphatic nodule?
Primary nodule: consists of only small inactive or naive lymphocytes
Secondary nodule: essentially an activated primary nodule which has been exposed to an Ag
What is a germinal center, and why is it lightly stained in slides?
The central region of the nodule and is lightly stained due to immature nature of cells
Define mantle zone (corona)
Outer ring of small lymphocytes that encircles the germinal center
What is aggregated lymphoid tissue and where is it located?
aggregated lymphoid tissue is essentially a massive amount of nodules in one location.
Can be located in tonsils, Peyer’s patches, veriform appendix, MALT
How does having aggregated lymphoid tissue in the tonsils aid the immune system?
Helps protect against inhaled or ingested substances
What type of tissue lines palatine tonsils? Does it possess crypts?
nonkeratinized stratified squamous epithelium with a partial capsule composed of dense CT.
What are the 3 types of tonsils?
Palatine, pharyngeal, and lingual tonsils
Where are pharyngeal tonsils located, and what distinguishes them from the other two types of tonsils?
Located on posterior wall of nasopharynx. They are covered by pseudostratified columnar ciliated epithelium and contain invaginations of mucosa, but no crypts
Where are lingual tonsils located, and what distinguishes them from the other two types of tonsils?
Positioned along the base of the tongue, covered by stratified squamous epithelium and they lack a capsule.
What are Peyer’s patches? What cell type are they lined with?
They are the numerous aggregations of lymphatic nodules in the small intestine, particularly the ileum.
They are covered by simple columnar epithelium with goblet cells
What is the vermiform appendix? What distinguishes it?
It’s an aggregated nodular tissue projection coming off of the cecum.
Characterized by crypts, but no villi
What is MALT?
Muscosa-associated lymphoid tissue (MALT) are clusters of lymphoid nodules named for the region/organ where they reside (e.g., GALT in GI or BALT in airways)
Define lymph
Clear, yellowish fluid comprised of interstitial fluid, solutes, and foreign materials.
Define lymphatic capillaries
Closed-ended tubes that are found among capillary beds
What are lymph nodes?
Small, encapsulated structures positioned along lymphatic vessels, comprised of reticular tissue, and are responsible for filtering lymph via removing debris, facilitating Ab production, and producing lymphocytes
What is the parenchyma of a lymph node?
The cortex and the medulla
What is the hilum of a lymph node?
The exit for efferent lymph and the entry/exit of neurovascular structures
Explain the flow of lymph in a lymph node
Afferent → superficial cortex → paracortex → medulla → efferent
What is the medulla of lymph nodes comprised of
Cords of lymphatic tissue called medullary cords which are separated by medullary sinuses
Where are both primary and secondary lymphatic nodules located in a lymph node?
superficial cortex
What is contained in the mantle zone of the superficial cortex
Active B cells
What comprises the deep cortex (paracortex)?
High % of T cells, and also it is free of nodules
Name the cells of the reticular meshwork.
Reticular cells (fibroblasts) responsible for secreting fibers and ground substance
Dendritic cells (APCs), macrophages, follicular dendritic cells.
What are high endothelial venules?
Postcapillary venules lined by cuboidal or columnar endothelial cells which facilitate movement of lymphocytes from circulation into the lymph node.
What is the thymus, and what happens to it as we enter adulthood?
Bilobed structure in the mediastinum where lymphoblasts invade and proliferate.
As we enter adulthood, the thymus undergoes involution (decreased activity of the thymus as it slowly becomes filled with adipose tissue
How can you distinguish thymic tissue tissue from others?
Thin CT capsule with trabeculae extending into the parenchyma. It lacks lymphoid nodules and germinal tissues
What occurs in the thymic cortex?
T cell maturation
What are epithelioreticular cells for
They provide the framework for T cell maturation in the thymic cortex
What are type I cortical epithelioreticular cells
Type I: located at the boundary of the cortex and the capsule between the cortical parenchyma and trabeculae. Consists of occluding junctions which create barrier that isolates developing T cells from the stroma
What are type II cortical epithelioreticular cells
Located in the cortex, contain desmosomes which allows joining of adjacent cells, and express MHC I and II which are involved in thymic cell education
What are type III cortical epithelioreticular cells
Located at the boundary of the cortex and medulla, they provide occluding junctions between processes of adjacent cells which creates a functional barrier between the cortex and medulla. They also express MHC I and II molecules
Why does the thymic medulla stain less intensely?
What is the distinguishing feature of the medulla?
This is due to the lymphocytes being much larger in the medulla comparatively.
Thymic corpuscles are a distinguishing feature
What are the 3 types of medullary epithelioreticular cells, and what do they do?
Type IV: Creates the barrier at the corticomedullary junction w/ type III cells
Type V: Compartmentalize groups of lymphocytes
Type VI: Thought to produce IL-4 and IL-7 that function in thymic differentiation and T-cell education.
Why is it important to have a blood-thymus barrier?
It protects developing lymphocytes in the thymus from exposure to Ags. If immature T cells were exposed to self-Ags prior to being fully matured, they could incorrectly target those self-Ags for destruction.
From the lumen outward, name the layers of the blood-thymus barrier
- Endothelium lining the capillary walls (highly impermeable)
- Macrophages within the perivascular CT which phagocytose all Ags that escaped from the capillary into the cortex
- Type I epithelioreticular cells which surround the capillary wall
What is the purpose of the spleen?
- Initiates immune response to blood antigens
- Reservoir for platelets
- Recycling center for erythrocytes
- Hematopoiesis
Distinguish between the white and red pulp of the spleen
White pulp: T and B cells along with macrophages all reside here
Red pulp: has venous supply, and contains RBCs, platelets, macrophages, and plasma cells
What tissue type is present in the spleen?
Dense CT capsule with trabeculae which contains myofibroblasts.
What’s the purpose of myofibroblasts within the spleen?
They help expel RBCs via contraction, and they produce extracellular fibers
Define the purpose of the PALS, MnZ, and MgZ within the white pulp of the spleen?
Periarteriolar lymphoid sheaths (PALS): T cells surrounding the central arteriole.
Corona/mantle zone (MnZ): Small lymphocytes surrounding GC
Marginal zone (MgZ): Blend of lymphocytes that span the white pulp into the red pulp
What is the purpose of the red pulp in the spleen?
It is involved in the blood filtration and erythrocyte turnover
What are stave cells? Where are they located?
Endothelial cells within the red pulp of the spleen that are elongated. They provide “fencing” to parcel out healthy vs. unhealthy RBCs
Beginning with the splenic artery, list the structures the blood will pass through on its way to, or within the spleen
Splenic A → trabecular As → central arteriole (PALS) → white pulp → penicillar As → splenic sinuses → splenic cords
