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
structure of the splenic sinuses
CONTAIN CAPILLARIES AND SINUSOIDS: dilated capillaries with discontinuous wall
-STAVE CELLS: long endothelial cells arranged with spaces between them which favours exposure to macrophages
BASAL MEMBRANE: fragmented and wrapped in rings around stave cells to form the openings
-SHEATH CAPILLARIES: surrounded by macrophages devoted to phagocytosis of aged erythrocytes
-CHORDS OF BILLROTH: cords of cells filling space between sinusoids and capillaries (macrophages, RBCs, WBCs) - reservoir
-sinusoids connected to the SPLENIC VEINS
How is hemocatheresis possible in the spleen?
new vs old erythrocytes distinguished via modification on the surface which makes old cells move less rapidly
!! causes tendendency to agglutinate, and hence are exposed due to open circulation for macrophages to engulf
Classifications of lymphoid tissue
-formed by lymphoid nodules that are aggregated and immersed in the walls of certain orgains
MALT - tunica mucosa
GALT- digestive (gut)
BALT- bronchial
2 special tissues:
-tonsils (oral cavity)
-peyer patches (ilium)
types of tonsils and the diff characteristics/ locations of each
- LINGUAL: stratified squamous non keratinised epithelium
- PHARYNGEAL: pseudostratified ciliated epithelium
- TUBARIC
- PALATINE
!! WALDEYER RING: the ring formed by all of the tonsils
CRYPTS: openings (seen as white spaces that increase the surface area) where antigens can enter and come into contact with lymphocytes
FOLLICLES: contained in the CT
Payer Patches location and function
LOCATION:
-marker of the ilium
-found in the tunica mucosa (but can be large enough to penetrate into the sub-mucosa)
!!! only present on one side of the ilium
FUNCTION: traps and destroys bacteria
What occurs in the wall of organs that posess lymph nodules?
DOME: area containing B cells, macrphages and plasma cells at the tip of the nodule
M (microfold) CELLS: intercalated between lining epithelia of the wall
-stimulate immune response by having microfolds for entrance of lymphocytes (favours antigenic contact)
HEVs: allow entrance of lymphocytes into nodule. Also allows exit of lymphocytes into efferent lymph vessels
