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Histology of the lymphatic system - Coggle Diagram
Histology of the lymphatic system
Types of immune cell
Lymphocytes (B and T)
Plasma cells - which secrete antibodies, derived from B cells
Macrophage- which engulfs microorganisms and presents antigens on its surface to lymphocytes
Dendritic cell- presents antigens to lymphocytes on its surface
Neutrophil- phagocytic cell
Reticular cell- also a dendritic cell, which presents antigens
Mast cell- derived from the bone marrow, release histamine, heparin etc. Have receptors for IgE antibodies on their surface. Involved in allergic reactions
B and T lymphocytes
Make up 20-50% of circulating leucocytes
Most are small, but about 3% are small and have more cytoplasm, these are mostly activated B lymphocytes which are on their way to tissues where they will become antibody secreting plasma cells. The activated lymphocytes can enlarge and have a basophilic cytoplasm, due to their increased protein synthesis
Two tyopes of lymphocytes are B cells and T cells
B cells are made and matured in the bone marrow
T cells are made in the bone marrow, but have to migrate and reside in the Thymus before they are matured
The two types of immune response are the Humoral antibody response, and the cell mediated immune response
Humoral antibody response: B cells and plasma cells make antibodies, helped by helper T cells. Mainly a response to infectious bacteria.
Cell mediated immune response - produced by killer T cells, which are cytotoxic. This is primarily a response to infectious viruses, foreign cells or fungi, which are antigenically different to the host.
B lymphocytes
Programmed to respond to a particular antigen, which stimulates the b cells if they are also stimulated by cytokines
activated B cells divide many times and produce lots of identical plasma cells (same antigenicity as the B cell). These B cells start to secrete antibodies.
Some of these B cells develop into plasma cells which are specialised to produce lots of antibody
The rest remain as small lymphocytes and persist in secondary lymphoid organs as long lived memory B cells, that can respond quickly when exposed to the same antigen
T lymphocytes
Formed in bone marrow but mature in the thymus
Make up 60 to 85% of the peripheral blood lymphocytes. They do not make antibodies, and the antigen receptors on their surfaces are different to those on B cells. They also have markers on their surfaces called CD antigens, which are expressed during maturation in the thymus. These also live in the peripheral lymphoid tissues.
There are 3 types of T cell
Cytotoxic T cells: destroy their targets by releasing perforin, which inserts into the lipid bilayer of the target cell, and polymerises into a large membrane channel, permeabilising the cell and killing it
Helper T cells: which regulate immune responses by releasing cytokines
Suppressor T cells, which downregulate both humoral and cell-mediated immune responses
Cytotoxic and suppressor T cells both express CD8, helper T cells express CD4 antigens. These types of cell cannot be distinguished using stains, u=but instead immunostaining for the different cell surface markers that they express.
Lymphoid tissues:
Lymph= clear fluid that flows in lymphatic vessels, lymphatic tissue and red bone marrow. Fluid filters out of capilleries and drains into lymphatic vessels to become lymph. The content of lymph is the same as tissue fluid (interstitial fluid).
Eventually lymph drains into venous blood. Lymph drains interstitial fluid, transports dietary lipids and facilities immune response.
Primary lymphatic organs
where lymphocytes are formed and mature. - allow stem cells to divide into B and T cells
There are two: red bone marrow and the thymus gland.
B and T cells are born in the bone marrow
B cells mature in the bone marrow, but T cells migrate to the thymus to mature.
Secondary lymphatic organs
These are arranged in a series of filters monitering the contents of the extracellular fluid e.g lymph, tissue fluid and blood.
The lymphoid tissue filtering out of each of these fluids is arranged in different ways.
This is also where lymphocytes are activated
These are the:
Lymph nodes
Tonsils
Spleen
Peyer's patches
MucosA associated lymphoid tissue (MALT)
Filtering lymph:
Lymph is filtered by lymph nodes, which are essentially encapsulated lymphoid tissue. There are 100-200 of these mostly in the neck, thorax, abdomen and pelvis
The lymph nodes contain B- and T-cells, which mostly enter the nodes via the blood stream, and also contains macrophages
Filtering tissue fluid:
Filtered by non-encapsulated aggregations of lymphoid tissue. They are usually around 1mm in diameter, but the Peyer's patches, tonsils and appendix are exceptions.
These aggregations are frequently found close to moist epithelial surfaces, e.g mucous membranes of the digestive, respiratory and reproductive systems
The epithelia of these tissues have mechanisms to keep bacteria out, but sometimes the bacteria gets in. Therefore the lymphoid cells in these areas are able to respond to any bacteria or micro-organisms. Activated B cells in these areas can develop into plasma cells and produce antibodies. Lymphocytes from the larger organs can respond quickly to foreign antigens around the surrounding tissue
Tonsils: patially encapsulated masses of lymphoid tissue, found in the walls of the pharynx and nasopharynx and at the base of the tongue.
Peyer's patches: Large masses of confluent lymphoid follicles, found in the walls of the ileum, part of the small intestine.
Filtering blood
Filtered by the spleen
The spleen is the body's largest lymphatic organ, and it is important for antibody production, facilitating immune responses to blood borne antigens, it is also eliminates worn-out blood cells and platelets.
The spleen is a large encapsulated organ in the left upper part of the abdomen, the outer capsule is fibro-elastic.
Tonsils
These are large non-encapsulated masses of lymphoid tissue, that lie in the walls of the pharynx and nasopharynx and at the base of the tongue
The luminal surface of the tonsils are covered with a stratified squamous epithelium.
The tonsils have many invaginations which form blind crypts.
Below the epithelium, there are many lymphoid follicles beneath which have germinal centres
Epithelial cells are able to phagocytose bacteria, and transfer them to macrophages, which then present the foreign antigens to B-cells, which are activated. The activated cells mostly secrete IgA type antibodies, which are secreted locally.
This is a photo of a tonsil
Spleen:
Largest mass of lymphatic tissue in the body, and is found between stomach and diaphragm.
Has a hilus (hilium) which is where the major blood vessels enter and leave.
Has efferent lymph vessels, which leave from the hilium, and it does not have afferent lymph.
Functions
A store for platelets
It reacts to blood borne antigens, by producing antibodies. It is the main source of circulating antibodies.
It removes defective red blood cells and platelets from the circulation.
Two types of tissue in the spleen
White pulp:
Contains lymphoid aggregations, mostly lymphocytes, and macrophages which are arranged around the arteries.
The lymphocytes are both T (mainly T-helper) and B-cells.
Red pulp:
This is vascular, and has parencyhma and lots of vascular sinuses. These are sinuosoids - a specialised type of capillary, which is very leaky.
Acting as a filter:
The lining endothelial cells have wide slits between their lateral margins, that act as a filter.
Blood cells have to move through these slits, before they can leave the spleen and worn out, or defective blood cells are damaged during this process.
The damaged cells are then phagocytosed by the numerous macrophages in the red pulp, that lie just next to the sinusoids.
The spleen is covered by a dense capsule, and there are connective tissue trabeculae, which provide internal support for the spleen, and carry the blood vessels into the spleen.
Mucosa associated Lymphoid Tissue and Peyer's Patches
MALT
The mucosa of the digestive, respiratory and urinary tracts often contains small aggregations of lymphocytes called lymphoid follicles. These are called 'Mucosa associated lymphoid tissue' (MALT).
MALT are sometimes large, and confluent. This happens in the tonsils, peyers patches and the appendix.
Epithelium of the mucosa contains special flat epithelial cells called M (membrane-like) cells or FAE (follicle associated epithelial cells).
Specialised cells take up small amounts of gut antigens entering the gut lumen, and pass them onto antigen presenting cells, and lymphocytes of the MALT.
Diffuse B-lymphocytes in gut react to micro-organisms that cross the mucosa, and they make IgA type antibodies, which are then secreted directly onto the gut lumen. These antibodies help to prevent micro-organisms in the gut from sticking to the gut epithelium, and can neutralise toxins and viruses.
Peyer's patches
Peyer's patches mostly contain T-cells, but also can have germinal centres that contain B-lymphocytes, as well as macrophages.
No afferent lymphatics.
The activated lymphocytes pass out in efferent lymphatics and travel to the lymph nodes.
Lymph nodes
What are they?
bean shaped organs found along lymphatic vessels, which filter micro-organisms from lymph
Covered by a capsule of dense connective tissue, and have capsular extensions of connective tissue, called the trabeculae
Trabeculae provide support for blood vessels entering into the lymph nodes
Lymph contains: micro-organisms, soluble antigens, antigen presenting cells, and a few B-cells,
Lymph enters the lymph node via afferent lymphatic vessels which enter the subcapsular sinus. It then runs through cortical sinuses into medullary sinuses and leaves through the efferent lymphatic vessel at the Hilium as efferent lymph.
Efferent lymph = contains lots of T-lymphocytes, B-lymphocytes, plasma cells and antibody.
All the blood sinuses are lined by a discontinuous layer of simple squamous endothelium, and they also contain lymphocytes and macrophages.
Reticular fibres provide additional support to the matrix/stroma.
The cortex is divided into an outer and an inner cortex.
The outer cortex has lymphatic nodules that mostly contain B-cells. Small lymphocytes sit in the spaces between the reticular fibre meshwork in the cortex.
The inner cortex contains mostly T-cells.
The deep cortical, and medullary cords contain B-cells and plasma cells.Plasma cells live for 3 days, and make IgG type antibodies.
The lighter staining areas are germinal centres, where the B-cells proliferate into antibody secreting plasma cells
Macrophages are also present in these regions, together with dendritic cells, and some T-cells. Both the macrophages, and the dendritic cells trap antigens and present them on their surfaces to B-cells.
Most of the lymphocytes enter the lymph nodes via blood vessels, and about 10% enter through the lymph.
The structure of the post-capillary venule, in the deep cortexis not lined by a simple cuboidal epithelium.
These are called high endothelial venules (HEVs)
Lymphocytes recognise and adhere to these endothelial cells, and squeeze through them into the deep cortical regions of the nodes. This region of the lymph has lots of T-cells, as well as the antigen presenting dendritic cells.
Thymus
primary lymphoid organ found within the superior mediatinum, behind the upper part of the sternum.
produces thymic hormones that support the growth and differentiation of T-cell progenitors.
active in children, but at the start of puberty, until old age, it starts to atrophy, producing fewer T-cells
two lobes divided up into many lobules.
The outer, more darkly staining region is the cortex, and this is highly cellular.
The inner lighter staining region is the medullar, and this region is less cellular. It has an outer connective tissue capsule and septa.
important for development of immunocompetent T-cells, proliferation of clones of of mature T-cells, developing immunological self-tolerance, and secretion of hormones for T-cell development.
three hormones are made: thymosin, thymulin and thymopoietin. These hormones are produced by reticular epithelial cells in the cortex.
The cortex stains more darkly (is more basophilic) than the medulla, because it contains more lymphocytes than the medulla.
The epithelial network in the cortex is more finely branched than in the medulla - and this gives this network the name 'reticular'.
The epithelial cells are connected to each other by desmosomes, and the intermediate filament protein keratin is present in their cytoplasm.
The immature lymphocytes only differentiate in the cortex.
The T-cell progenitors proliferate in the outer cortex.
However, only 5% of the T-cells formed survive. The survivors are the ones that can recognize self-MHC encoded surface glycoproteins, i.e will be immunotolerant to antigen presenting cells from the same person.
Differentiating T-cells accumulate between epithelial reticular cells. They can then pass into the venules and efferent lymphatics along the border between the cortex and the medulla. Or they pass into the medulla, where they are further selected by thymic dendritic cells and matured before passing out of the medullary venules and efferent lymphatics.
Blood from the inferior thyroid, and internal thoracic arteries enters the thymus. The epithelial cells form a sheath around the capillaries, to form a barrier to the entry of antigenic material into the spaces between the epithelial cells in the reticular network. This is known as the blood thymus barrier.
Further, the thymus only has efferent lymphatics, not afferent lymphatics, to transport lymph and lymphocytes away from this organ.
