Immune System pt.2 (Secondary Lymphoid Tissue) Flashcards
What is the function of secondary lymphoid organs?
immune system function
- immune response
- do not produce new cells!
What is the most important secondary lymphoid organ?
lymph node
Where are lymph nodes located?
scattered throughout the body
Where does lymph flow? What happens to it?
- flows in from body tissues and organs
- filtered by the node
What occurs at the lymph node?
- lymph filtration
- lymphocyte production
- antigen presentation and recognition
What do lymph nodes contain? What are they?
- multiple lymphatic nodules (aggregates of lymphocytes)
- macrophages
- reticular cells
- follicular dendritic cells
- other immune cells
What is a lymphatic nodule?
Lymphatic nodules are small, localized, oval-shaped aggregations of lymphoid tissue that lack a connective tissue capsule. They contain clusters of lymphocytes, primarily B-cells, along with some T-cells and antigen-presenting cells. Unlike lymph nodes, they are not surrounded by a fibrous capsule and are found within mucosal tissues.
What forms the framework of the node? What is its function?
- reticular fibers (produced by reticular cells)
- form node framework
- create a filtering mesh-work in the sinuses
What surrounds the lymph node?
capsule
- dense irregular connective tissue
In most species, afferent lymphatics enter the node at sites along the capsule and release lymph into what part of the node? Where is it located?
- subcapsular sinus
- immediately beneath the capsule
State the direction of lymph flow.
subscapular sinus –> cortical sinuses –> medullary sinuses
What are the sinuses of the lymph node lined by?
endothelial-like reticular cells
What spans the sinus lumen? What is its function? Why?
- stellate reticular cells
- slow the flow of lymph
- allow for more interaction between lymphocytes and antigens in the node
What is the subscapular sinus?
- not empty space!
- 3 dimentional network of reticular connective tissue
What regions do lymph nodes possess?
cortex (outer region)
medulla (inner region)
Can you see the lymph node sinuses?
no! they are fully hypothetical. there are no borders distinguishing them (subscapular, cortical, medullary)
What are the important features of the lymph node?
- the entire lymph node is filled with 3D network of reticular connective tissue
- zones are hypothetical
- sinuses are not entirely filler with empty space
What does the lymph node cortex contain?
multiple lymphatic nodules seperated by cortical sinuses
What do active lymphatic nodules have?
- light staining germinal center (development center)
+ large B-lymphocyte - surrounding follicular dendritic cells
What are lymphocytes in the germinal center capable of doing?
active proliferation
What is the function of follicular dendritic cells?
- support B cells
- assemble cells into follicles
What is the germinal layer surrounded by? What is it comprised of?
- dark staining corona (mantle)
- comprised of: small lymphocytes (B-lymphocytes or memory B-cells) in the process of migrating from the germinal layer
What is the primary cell present in lymph nodes?
B-lymphocytes
What are the two main parts of the lymphatic nodule?
developmental center: germinal center (filled with a large B-lymphocyte with proliferative capacity)
corona: cells which already divided their germinal center.
What causes B-cell proliferation?
- activation
- exposure to antigens (antigen-preseting cells (dendritic cells))
What cells present their antigens?
dendritic cells
What happens when B-cells are exposed to dendritic cells?
B-cells express surface immunoglobulins and become either B memory cells or plasma cells which then leave the nodule.
What do activated B-cells turn into?
- B-memory cells
- plasma cells
What are cortical sinuses? What are they continuous with?
- extend between the nudules from the subcapsular sinus to the medulla
- continuous with medullary sinuses
What cells are dendritic cells similar to? What do they do?
macrophages
- do not fight cells!
- simply digest them, go to the nearest lymph node, present the antigen to B-lymphocytes
What is a hypothetical zone of the lymph node? Where is it located?
paracortex (innercortex)
- junction between the cortex and medulla
What does the paracortex represent?
site of T-cell maturation
What cells are present in the paracortex?
- few lymphatic nodules
- reticular cells
interdigitating cells
What is the function of interdigitating cells?
- migrate from different body regions into the paracortex
- present antigens to T-cells
- t-cell moves to the medullary sinuses and exits the node
Why is the lymph node paracortex important?
location of T-lymphocyte accumulation
What lymphocytes are in the lymph node? Where?
B-lymphocytes (cortex of lymph nodules)
T-lymphocytes (paracortex, between medulla and cortex)
How does the lymph node medulla compare to the cortex?
- less cellular
- stains lighter
lacks lymphatic nodules (expt. pig)
What is special about the pig lymph node?
lymphatic nodules present only in the medulla but not cortex.
How is the medullary region of the node arranged?
- cellular medullary cords
located between medullary sinuses
What cells are included in the medullary lymph cords?
- B-memory lymphocytes
- T- lymphocytes
- plasma cells
- macrophages
- reticular cells
What cells form the framework of the lymph node cellular medullary cords?
reticular cells along with reticular fibers
What are medullary cords?
- new concept
- aggregations of many different cells
- combined into linear structures
- kept together by reticular fibers and cells
What are plasma cells?
- activated stage of B-lymphocyte
- already producing antibodies
What happens to immune system cells (T/B-lymphocytes) when they are activated?
They migrate from the lymph node to another lymoh node via blood circulation.
What is the function of the spleen?
- filters and stores blood
- participates in blood cell formation in the fetus
- removes spent erythrocytes
- mounts an immune response against blood-borne antigens
State the composition of the spleen.
- lymphatic nodules are scattered throughout the parenchyma
- no cortex
- no medulla
How is the spleen organised?
- red pulp: blood filled sinuses
- white pulp: cords of splenic cells (large numbers of lymphocytes)
Which pulp is abundant in which part of the spleen?
white pulp = defensive spleen
red pulp = storage spleen
What are the two main functions fo the spleen?
- immune system organ
- blood reservoir
Why does the spleen red-pulp look paler than the white-pulp?
spleen is empty in the picture, and therefore not filled with red blood.
What is the major difference of the capsule of the spleen?
- it can contract
- composed of smooth muscle cells
(connective tissue trabeculae extend from the capsule into the parenchyma)
contracts to release blood into the blood stream
What is a practical fact about the capsule?
very sensitive to drugs eg. anaesthesia –> capsule expands –> spleen engorges with blood
What forms the red pulp of the spleen?
- splenic sinuses
- cellular splenic cords
What are splenic sinuses lined by? What can they do?
- longitudinal endothelial cells
- contract to allow gaps to form between cells
What creates a supporting structure for the sinus wall?
reticular fibers
- surround the fenestrated basement membrane of the endothelium
What does red pulp contain?
- framework of splenic sinuses (sinusoidal capillaries)
- splenic chords
How does the spleen accumulate blood?
Blood travels in the sinusoidal capillaries, exits into the spleen.
What are splenic cords similar to?
medullary cords
What is the function of splenic cords?
form a network around the outside of the sinuses
What are splenic cords composed of?
- erythrocytes, macrophages, plasma cells, and lymphocytes
- framework of reticular cells and fibers
In what pattern do reticular fibers hold all cells together in splenic cords?
linear pattern!
What is the white pulp of the spleen comprised of?
- periarterial lymphatic sheats (PALS) (T-cells)
- lymphatic nodules + associated efferent lymphatics (B-cells)
What forms the supporting network for the white pulp?
reticular cells and fibers
What are PALS? What do they surround?
periarterial lymphatic sheats
- sheaths of T-cells
- surround arteries passing into the parenchyma from the connective tissue trabeculae
Where do T-lymphocytes predominate?
- adjacent to the tunica media of the vessel
What is scattered along the blood vessels within the white pulp?
lyphatic nodules (may/may not have active germinal centers)
What may be present in lymohatic nodules?
germinal centers
What predominates in lymphatic nodules?
B-lymphocytes
Which cells’ location heavily differs between the spleen and lymph nodes? Why?
T-lymphocytes are located in ‘sleeves/rings’ (PALS) periarterial lymphatic sheats (spleen) or the paracortex (lymph node).
What happens to the spleen and lymph nodes when the body undergoes an infection?
the immune system organs increase in size
What is a common outcome of infections?
splenomegalia (inflammation of the spleen)
How does blood flow into the spleen?
via the splenic artery
What is the trabecular artery known as, when it emerges from connective tissue? What is it surrounded by?
known as: artery of the white pulp
surrounded by: PALS
