Cells & Organs of Immunity Flashcards
what does the lymphatic system do?
what do lymphocytes react to?
what are the primary immune organs responsible for the synthesis and maturity of lymphocytes?
where are B &T lymphocytes made?
where do T cells migrate to mature?
what happens after B & T lymphocytes become immunocompetent?
where do secondary immune organs tend to be located?
where are lymph nodes located?
where is the spleen located?
where are mucosa associated lymphatic organs located?
The lymphatic system mediates immune responses against foreign invaders (such as viral or bacterial cells). Lymphocytes react to pieces of foreign cells and molecules called antigens and then activate an antibody- or cell-mediated response that ultimately results in the destruction of the foreign invader.
The primary immune organs are responsible for the synthesis and maturation of lymphocytes, and include the bone marrow and the thymus.
Both B- and T-lymphocyes are made in the bone marrow.
B-cells also mature in the bone marrow.
Although T-cells are made in the bone marrow, they then migrate to the Thymus where they mature and become immunocompetent- capable of recognizing and responding to foreign antigens.
After B- and T-lymphocytes become immuno-competent, they circulate through the lymphatic and blood vasculature, and/or become resident in secondary lymphatic organs, where they activate immune responses against antigens.
Secondary immune organs are dispersed throughout the body but tend to be located where antigens are likely to be concentrated.
For example, Lymph Nodes are located throughout the lymphatic vascular system, through which antigens circulate, and the Spleen is located in the abdominal cavity where it receives a large blood supply.
Mucosa-associated lymphatic organs are located along the respiratory tract (the tonsils), and in the digestive system (Peyer’s patches).
where is the thymus located?
when is the thymus functional? what happens to it during puberty?
how is the thymus composed?
what does each lobe of the thymus contain?
what does the connective tissue do in the thymus?
what does the trabeculae do?
where are most of the T-cells in the thymus concentrated in?
where do T cell mature in the thymus?
what does maturation of T cells involve?
what happens to T cells that can’t recognize MHC molecules?
what if T cells recognize self-proteins as foregin?
what do immature T cell mitigate?
while T cells mature where do the migrate to?
where do 98% of T cell die? how does this happen?
what happens to surviving Tcells?
The thymus is located in the thoracic cavity, above the heart and extends over the vessels of the heart.
The thymus is present and functional during development but usually involutes, or atrophies during puberty.
The thymus is a small organ composed of two distinct lobes, and encapsulated in a connective tissue capsule of dense irregular collagenous connective tissue.
Each lobe of the thymus contains a cortex and a medulla.
A connective tissue capsule surrounds the cortex and extends trabeculae into the cortex.
The trabeculae subdivides the cortex and medulla into incomplete lobules.
Maturation of T-cells in the thymus:
Most of the T-cells in the thymus are concentrated in the cortex (This is why it stains darker than the medulla of the thymus).
T-cells mature in the cortex of the thymus. Maturation of T-cells involves exposure of T-cells to self-MHC molecules and self-protein epitopes.
T-cells that cannot recognize MHC molecules undergo programmed cell death or apoptosis.
In addition, T-cells that recognize self-proteins as foreign (and thus potentially could initiate an auto-immune reaction) will undergo apoptosis.
Immature T-cells first migrate to the periphery of the cortex, and then proliferate and become immunocompetent.
While T-cells are maturing, they progressively migrate to the inner cortex- towards the corticomedullary junction.
98% of developing T-cells die in the cortex, and are phagocytosed by macrophages there (probably because they recognize self-proteins).
The surviving T-cells exit the thymic cortex via the vasculature.
why is understanding the vascularization of the thymus important?
how do blood arteries enter the thymus?
how far do the arteries entend through in the cortex?
how do T cells leave the capillary beds?
as t cells mature where do they move to?
what path do the veins have that is similar to the arteries?
Vascularization:
Understanding the vascularization of the thymus is important to obtain a clear image of the path of migration of T-cells through the thymus.
Blood arteries enter the thymus through connective tissue trabeculae that penetrate the cortex region of the thymus.
The arteries extend through the cortex to the medulla and then loop back towards the cortex where they form capillary beds.
Immature T-cells leave the capillary beds in the cortex, and migrate up towards the periphery of the cortex, where they begin the maturation process.
As T-cells mature, they move to the inner cortex and medulla and then exit through veins draining the capillary network of the thymic medulla.
The veins traverse the same path as the arteries: through the medulla and then looping upward along trabeculae and out of the thymus
what is the function of epithelial cells?
where are the different epithelial cell types located?
what do some epithelial cells make?
what can epithelial reticular cells form ?
what can epithelial reticular cells and capillary endothelial cells form?
what is the blood-thymic barrier permeable to?
what is another function of epithelial reticular cells?
Epithelial Reticular Cells:
Blood thymus barrier
Epithelial reticular cells are a supportive cell type (intermediate between fibroblasts and epithelial cells) in the thymus.
Epithelial reticular cells comprise several types and are located in both the cortex and the medulla of the thymus.
Some epithelial reticular cells make up the blood-thymic barrier that prevents immature T-cells from coming in contact with foreign antigens.
Epithelial reticular cells form a sheath along the capillaries that enter the thymic cortex.
Epithelial reticular cells together with the capillary endothelial cells form a basement membrane that makes the capillary very impermeable.
The blood-thymic barrier is permeable to self-proteins, and to immature T-cells (so that maturing T-cells can be tested for a response to self-proteins).
Another function of epithelial reticular cells is to present MHC molecules to T-cells.
what are signature features of Hassals corpscles?
what do the Hassal’s corpuscles consist of?
what are Hassal’s corpuscles thought to be the site of?
when do Hassal’s corpuscles increase?
what happens to the thymus during the first years of life? how does this process work?
what can occur even after the thymus has involuted?
Hassal’s corpuscles and involution of the thymus
Hassal’s corpuscles are a signature feature of the medulla of the thymus.
Hassal’s corpuscles consist of whorls of epithelial reticular cells that have coalesced around one another. Hassal’s corpuscles are thought to be the site of T-lymphocyte cell death in the medulla (i.e. T-cells that either recognized self-proteins, or could not recognize MHC molecules).
Hassals’ corpuscles increase in number and size with a person’s age.
After the first few years of life the thymus begins to involute and atrophy.
This is a normal process that involves the thymus becoming infiltrated by adipose cells, and an increase in size and number of Hassal’s corpuscles.
Even after the thymus has involuted, it is still capable of making mature T-cells.
where do B&T mature lymphocytes travel to?
where are lymph nodes located?
what do lymph collect?
what is the main function of lymph nodes?
what are the cells found in lymph nodes ?
what do lymph nodes consist of?
what is this capsule like?
what does the cortex region of the lymph node contain?
what does the medulla of the lymph nodes contains?
what else do lymoh nodes also contain?
what do the paracortex contain?
Once B- and T-cells are mature and immunocompetent, they travel via the blood and lymphatic vasculature to secondary immune organs where they initiate immune responses against foreign invaders of the body.
Lymph Nodes
Lymph Nodes are located along the lymphatic vasculature of the immune system.
Lymph collects from the tissues of the body, and flows through the lymphatic vasculature.
The main function of lymph nodes is to filter lymph, removing or destroying the foreign antigenic material in lymph.
The cells found in lymph nodes include T- and B-lymphocytes, macrophages and other antigen presenting cells.
Lymph Nodes consist of a capsule surrounding a cortex and medulla region.
The capsule is dense irregular collagenous connective tissue that extends trabeculae into the cortex of the lymph node.
The cortex region of the lymph node contains follicles or nodules, which are conglomerations of reactive B-lymphocytes.
The medulla of the lymph nodes contains cords: loose aggregates of mixed cells interspersed within lymphatic sinuses.
Lymph nodes also contain a third region called the paracortex which is defined the region between the cortex and the medulla plus the tissue in the cortex around the follicles.
The paracortex contains mainly T-lymphocytes
what are lymph follicles?
how do the follicles become stimulated?what will this result in? and what will develope?
what will be found in the corona region of the stimulated follicle ?
why is the formation of the germinal center important?
where are T-cells located?
what do T-cells surround?
how are T and B cells distinct?
what do cellular cords contain?
what do the cellular cords surround?
what are sinuses lined by?
Lymphoid Follicles (Nodules)
Lymphoid follicles are dense aggregations of B-cells (B-lymphocytes).
In response to antigenic exposure, follicles become stimulated or activated.
This will result in extensive proliferation of the B-cells, and reorganization of the follicle.
Specifically, a lighter-staining germinal center will develop in the follicle.
The germinal center will contain proliferating B-lymphoblasts (which stain lighter).
The corona region of the stimulated follicle around the germinal center will contain B-lymphocytes.
The formation of a germinal center is a key step in B-cells mounting an immune response against the antigen.
T-cells are located in the paracortex region of the lymph node.
T-cells surround the B-cell rich follicles.
Thus, T- and B-cells have distinct distributions in the cortex and paracortex of the lymph node
The cellular cords in the lymph node medulla also contains some T- and B-cells, but at a much lower concentration than in the cortex and paracortex, and in a much different organization than in the cortex.
The cellular cords surround an extensive network of sinuses (through which lymph will leave the node). Sinuses are capillaries lined by endothelial cells with large spaces or hole between them
how does lymph enter the node?
what is the function of these lymphatic vessels?
what does lymph get collected?
where does lymph that flows out of the sinuses go to?
when lymph exists the cortex and paracortex where does it go to ?
where does lymph then get collected to?
what is the word percolate used to describe?
Vasculature of the lymph node and the flow of lymph through the node
Lymph enters the node through afferent lymphatic vessels that pierce the convex side of the lymph node. These vessels have valves to keep the lymph flowing in the appropriate direction towards the lymph node.
Lymph then collects in the subcapsular sinuses and the trabecular sinuses.
Lymph flows out of these sinuses into the cortex and paracortex where the T- and B-cells act on the lymph- react to antigenic material.
Lymph drains from the cortex and paracortex into the medullary sinuses.
Lymph collects into the efferent lymphatic vessels at the hilum of the lymph node, and progresses to the next lymph node.
The word percolate is often used to describe the flow of lymph through the lymph node: very slow movement of lymph through the highly cellularized regions of the node, eventually into the medullar sinuses, and the efferent lymphatic vessels.
what is the connective tissue mechanism that supports the highly cellular organization of the lymph node?
Supportive connective tissue of the lymph node
What is the connective tissue mechanism that supports the highly cellular organization of the lymph node? The follicles of the lymph node, as well as the medullary cords and sinuses are all positioned correctly by an extensive network of reticular fibers- thin, collagen fibers that are made by reticular cells located throughout the node. Reticular cells are fibroblast derived, and synthesize the type III collagen that makes up the reticular fibers
what is the spleen involved in?
what else does the spleen function as?
what does the spleen not have?
how is the spleen divided?
in stained histology sections how does the red pulp look like?how does white pulp look like?
what is the spleen consist of?
Spleen
The spleen is involved in filtering blood borne pathogens, rather than lymphatic pathogens. The spleen also functions in destruction of aged red blood cells. The spleen does not have a cortex and medulla. Rather, the spleen is divided into red pulp and white pulp areas. (The colors red and white are based on how these two regions appear in the live tissue.) In fixed and stained histology sections, the red pulp appears light pink or less dense, while the white pulp appears darker due to the high concentration of lymphocytes and follicles there. The spleen (pizza) largely consists of red pulp (tomato sauce) with white pulp (pepperoni) embedded in it
what do the white pulp regions of the spleen consist of?
what is the sheath of T-lymphocytes called?
what is present within PALS?
how are these lymphatic follicles identified?
To summarize what does white pulp consist of?
how will white pulp appear like?
what can PALS be thought as?
what do the red pulp consist of?
what will the sinuses appear like?
what do the cords of cells surrounding the sinuses consist of?
The white pulp regions of the spleen consist of a sheath of dense T-lymphocytes surrounding a central artery (a branch of the main splenic artery).
The sheath of T-lymphocytes is called the PALS (peri-arteriolar lymphatic sheath).
However, within the PALS, lymphatic follicles of B-cells may be present.
These follicles are usually identified by their darker corona and paler germinal center.
To summarize, the white pulp consists of PALS (possibly with embedded follicles) surrounding a central artery.
The white pulp will appear darker in our sections due to the density of lymphocytes in the PALS and follicles.
Note that the PALS (T-cells) can be thought of as being analogous to the T-cell rich paracortex of the lymph node.
The red pulp (the body or parenchyma of the spleen) consists of sinuses surrounded by cords of lymphocytes (cords of Billroth).
The sinuses may appear like white spaces or they may be filled with red blood cells, or even with a few lymphocytes.
The cords of cells surrounding the sinuses consist of various types of lymphocytes and other white blood cells.
how does blood flow in the spleen?
where does the splenic artery enter?
how will the central arteries of the white pulp branch off to?
what do the penicillar arteries do?
how does blood get drained from the spleen?
Vasculature
Blood enters into the white pulp of the spleen, progresses from the white pulp to the red pulp of the spleen, and then exits the spleen.
In more detail, the splenic artery enters the spleen and branches into many central arteries (in the white pulp).
The central arteries of the white pulp will branch into the penicillar arteries that enter the red pulp.
Penicillar arteries drain blood into sheathed capillaries (sheathed by macrophages) in the red pulp.
Blood then drains from the sheathed capillaries of the red pulp into:
a) Cords of cells (of Billroth) in the red pulp. Blood would then collect from the cords of cells in the red pulp into venous sinuses in the red pulp.
b) Directly into venous sinuses of the red pulp.
In either case, blood will drain from the venous sinuses to central veins and ultimately to the splenic vein that exits the spleen
Redraw the table of the organ, if it has a cortex/medulla, if it has follicles, and any other notes on its organization.
Organ | Cortex/Medulla | Follicles | Other
Thymus | Yes | No | Only T-cells, Hasall’s corpuscles in medulla
Lymph Nodes | Yes | yes (in cortex)
| Cords of cells in medulla
Spleen | No | yes (within PALS) | Red pulp/white pulp
PALS in white pulp
