Lymph organs/tissue Flashcards
What are the components of the lymphatic system (7)
- bone marrow
- thymus
- spleen
- lymph nodes
- tonsils
- general lymphatic tissue
- peyer’s patches
What is the main diff between lymphoid organs and tissue
organs are delimited by a capsule of CT whereas lymphoid tissue is made of groups of lymphoid cells (forming follicles) that do NOT have a CT capsule
2 types of lymphoid organs
PRIMARY: BM & Thymus
- site of B and T cell production
-BM produces B cells and pre T cells
-Thymus produces T cells from pre-T cells
SECONDARY: lymph nodes/spleen/lymph nodules
-site of most immune responses
-site of antigen contact with lymphocytes
-mainly made of B and T cells
concentration of lymphocytes within diff organs of lymph system
- BM: 90% B cells
- Thymus: 100% T cells
- Blood: 70% T cells
- Spleen: 55% B cells
- Lymph nodes: 60% T cells
route of travel between lymph organs of the B and T cells
- PRIMARY ORGANS: maturation of lymphocyte progenitors to produce B cells in BM and T cells in thymus
- Migration of B cells and T cells into secondary organs
- B cells proliferate and differentiate to form plasma cells and aid in humoural response
- T cells mature and help in cell mediated response
Thymus functions (3) and location
LOCATION: above heart, posterior to sternum
FUNCTIONS:
1. site of T cell maturation via negative and positive selection (which then migrate to blood)
2. secrete hormone THYMOSIN which stimulates T cell maturation
3. destruction of incorrect T cells by macrophages
general structure of the thymus
-CT capsule with extending trabeculae (contains bood vessels)
-Trabeculae split thymus into lobules
-each lobule has a cortex and an inner medulla
-100% T lymphocytes that give it a basophilic affinity
what is the relative staining intensity of thymus cortex vs medulla and why
basophilic extent of cortex is greater than that of medulla
REASON: medulla has cells that are less densely packaged
Cells present in the cortex of the thymus (3)
- Epithelioreticular cells (ERs) type 1 to 6
- thymocytes (T cells)
- macrophages – allow digestion of incorrect T cells
Cells present in the medulla (5)
- Epithelioreticular (ER)
- Thymocytes
- Macrophages
- dendritic cells
- Hassal corpuscles (made of ER type 6)
Role of all the types of epithelioreticular cells in the thymus
1 - make the thymus blood barrier within cortex (blocks antigens from coming into contact with T cells during maturation)
2 - act with T cells for positive selection
3/4 - form barrier between cortex and medulla
5- contact with T cells in medulla that stimulate T cell differentiation
6- form medullar hassal corpuscle that releases cytokines to help T cell differentiation
what are the three elements of the thymus blood barrier
ER type 1
macrophages
capillary endothelium and basal lamina
!! no lymphatic vessels are present so that antigens do not come into contact with the maturing t cells
how are T cells determined to be correct or incorrect?
LOCATION: THYMUS
+ve selection: in cortex, T cells that can recognise antigens mature
-ve selection: in medulla, T cells that CANT recognise SELF antigens move into capillaries (others are phagocytosed by macrophages)
How does the thymus age over time
-peak during childhood and undergoes regression past puberty
-reduced in size
-parenchyma replaced by adipose tissue
lymph nodules location and function
LOCATIONS: neck, inguinal area, abdomen –> positioned along lymphatic vessels
FUNCTIONS:
-filters lymph before it returns to blood
-contains defense cells and has B/T cells that respond to antigens
General morphology of the lymph nodes
- CT capsule + trabeculae
-marginal sinus (lymph brought in by lymph vessels)
-lymphocytes are the main cell of the parenchyma
-superficial cortex containing follicles with an outer mantle and a germinal center
-paracortex (deep) containing high endothelium vessels (HEVs)
-medulla made of lymphatic sinuses
-hilum = place of concentration of ducts/blood and lymph vessels leaving node
Cells of the lymph nodules (7)
- B cells (unstimulated in mantle and stimulated in germinal center of follicles)
- T cells in deep cortex
- Macrophages in medulla
- dendritic cells
- reticular fibroblasts (produce reticular fibers)
- follicular dendritic cells (FDC) that bind antigens to B cells and aid B cell differentiation
- Plasma cells in medulla
what is the role of HEVs in the lymph node?
-entrance of lymphocytes into nodules
-circulation of lymph
-migration of T/B cells to underlying lymphatic tissues mediated by certain adhesion molecules and chemokines
Spleen functions (6) and location
LOCATION: left side of stomach and the biggest lymph organ
FUNCTIONS:
IN FETUS: haematopoietic, forming RBC (but this is lost post birth)
IN ADULT:
-immunological filter of blood (removes antigens)
-removes Ag
-acts as blood reservoir
-hemocatheresis: destruction of aged erythrocytes
General morphology of spleen
-CT capsule containing myofibroblasts + septa
-splenic artery enters at hilum
-branching of splenic artery to form the CENTRAL artery and wrapping of these branches in layers of lymphocytes –> PALS (peri-arterial lymphatic sheath)
-PALS enlarge and become rounded (accummulating B cells aggregates) –> Malphghian corpuscles
-branching of artery past the MCs forms penicilalr arterioles
-sheath capillaries surrounded by macrophages
-connection of sheath capillaries with sinusoids, then connected with splenic vein
2 types of pulp in the spleen and the components of each type
20% WHITE PULP: central artery, periarterial sheath and malpighian corpuscle
IMMUNE RESPONSES
80% RED PULP: sinusoids, blood cells and macrophages
ERYTHROCATHERESIS
!! these are mixed throughout the spleen
similarities difference between lymph node follicle and cross section of PALS
similarity: both have a germinal center
difference: PALS contains the central artery that is covered by the lymphocyte sheath whereas follicles are not vascularised internally
cells of the spleen
-B and T cells
-Red pulp macrophages
-APCs
-STAVE CELLS: endothelial cells lining the splenic sinuses
2 mechanisms by which blood flows from the penicillar arterioles into the sinusoids and then to capillaries
- NOT IN HUMANS: through closed circulation where the capillaries and sinusoids are connected (hence direct blood flow)
- METHOD IN HUMANS:
open circulation with no connection between sheath capillaries and sinusoid. Blood exits into open space, aged erythrocytes have trouble move through fenestrations. Macrophages ready to engulf the exposed (old) RBCs
