Lymphoid system The lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph (from Latin lympha"water"
Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils.
The system also includes all the structures dedicated to the circulation and production of lymphocytes, which includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system.
Classification of lymphocytes B cells (B = bone) T cells (T = thymus) NK cells (NK = Natural Killer)
B cells Synthesizes IgM and IgD Activated and proliferated by helper T-lymphocytes Performs the role of antigen-presenting cells (APC) Bone marrow
T cells Cell-mediated immunity Has T cells receptors Bone marrow thymus Function changes per subset
NK cells Produced in the red bone marrow Releases small cytoplasmic granules of proteins Target cell = die Apoptosis – programmed cell death Recognizes general classes of cells Involved in innate immunity
Primary lymphoid organs The central or primary lymphoid organs Generate lymphocytes from immature progenitor cells. Includes thymus and bone marrow
THYMUS Gk. “thumos”, meaning heart, soul, desire, life. A specialized organ of the immune system. A two lobed, glandular organ It is largest after birth and during childhood. Is replaced by adipose (fat) tissue when one gets older
THYMUS pinkish-gray color, soft, and lobulated on its surfaces in children. It is yellowish in adults At birth it is about 5 cm in length, 4 cm in breadth, and about 6 mm in thickness. Consists of two lateral lobes
HISTOLOGY OF THYMUS Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue Entire organ enclosed enclosed by a thin connective tissue capsule from which numerous septa extend into the thymus subdividing the two lobes into numerous lobule.
HISTOLOGY OF THYMUS Primary lobules vary in size from that of a pin's head to that of a small pea, and are made up of a number of small nodules or follicles. The follicles are irregular in shape and are more or less fused together, especially toward the interior of the organ. The cells housed within the thymus are primarily white cells called T lymphocytes, along with smaller amounts of other lymphoid cells.
THYMUS Each lobulus is divided into a darker peripheral zone, the cortex, and a lighter, central zone, the medulla. The cortex and medulla play different roles in the development of T-cells
Thymus: cortical portion The darker, peripheral zone Composed mainly of lymphoid cells supported by a network of finely-branched epithelial reticular cells Reticular cells and macrophages are present in addition to the lymphocytes, which are the dominant cell type within the lobules. The cortex is the location of the earliest events in thymocyte development, where T cell receptor gene rearrangement and positive selection takes place.
LOBULES AND INTERLOBULAR CONNECTIVE TISSUE,cortex and medulla
reticulaRcELLS “Thymic epitheliocytes.” Quite abundant. Cytoplasm is eosinophilic, and their large, ovoid and light nuclei may contain one or two nucleoli. Their slender processes are connected with the processes of other reticular cells to form a cellular reticulum (or cellular network).
reticulaRcELLS Reticular cells sheathe the cortical capillaries; they form an epitheloid layer which delimits the cortical tissue from the connective tissue and secrete substances (hormones and other factors) important for thymic function. They create and maintain the microenvironment necessary for the development of T-lymphocytes in the cortex.
Macrophages and lymphocytes Macrophages occur in both cortex and medulla. They are difficult to distinguish from the reticular cells Lymphocytes are present in both cortex and medulla, but are more numerous in the cortex. Their sizes are variable (5 - 15 µm) in the cortex but generally small in the medulla. The vast majority of them will be developing T-lymphocytes. They are also called thymic lymphocytes or thymocytes.
Thymus: medullary layer Lighter, central zone The reticulum is coarser than in the cortex, the lymphoid cells are relatively fewer in number Hassall's corpuscles are present location of the latter events in thymocyte development.
Hassal’s corpuscles “Thymic corpuscles” Peculiar nest-like structures found in the medulla of the human thymus Composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells. are rounded eosinophilic structures, which consist of concentrically arranged, flattened cells.
Hassal’s corpuscles Thymic corpuscles are likely to be formed by reticular cells Increases in size as one gets older The function of Hassall's corpuscles is currently unclear
Involution of the thymus After puberty, much of the parenchyma of the thymus, cortical lymphoid in particular is replaced by adipose tissue This process is called involution initially proceeds rapidly but slows down in adulthood. It is under the control of steroid hormones It is a common feature of all lymphoid tissues but most pronounced in thymus
Vasculature The arteries supplying the thymus are derived from the internal mammary, and from the superior thyroid and inferior thyroids. The veins end in the left brachiocephalic vein (innominate vein) , and in the thyroid veins. The nerves are exceedingly minute; they are derived from the vagi and sympathetic nervous system. Branches from the descendens hypoglossi and phrenic reach the investing capsule, but do not penetrate into the substance of the organ.
thymus The main function of the thymus gland is to mature, or "educate" antibody producing white cells called T lymphocytes In the two thymic lobes, hematopoietic precursors from the bone-marrow, referred to as thymocytes, mature into T-cells. Loss of the thymus at an early age through genetic mutation (as in DiGeorge Syndrome) results in severe immunodeficiency and a high susceptibility to infection. Its function is diminished in adults but still continues its endocrine function
BONE MARROW Flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells (hematopoiesis) Produces lymphocytes and acts to prevent the backflow of lymph. Constitutes 4% of the total body mass of humans Cone marrow hematopoietic compartment produces approximately 500 billion cells per day
BONE MARROW RED MARROW “Medulla ossium rubra” Consist mainly of hematopoietic tissue For formation of RBCs, platelets, and most WBCs Found mainly in flat bones and cancellous material at the epiphyseal ends of long bones YELLOW MARROW Consist mainly of fat cells Found in the medullary cavity, the hollow interior of long bones
Bone marrow Both types of bone marrow contain numerous blood vessels and capillaries. All bone marrow is red at birth In cases of severe blood loss, the body can convert yellow marrow back to red marrow to increase blood cell production.
Bone marrow in femur head showing red and yellow marrow
Bone marrow: stroma Stroma are cells that support the function of the parenchymal cells of an organ. The stroma of the bone marrow is composed of all the tissue not directly involved in the primary function of hematopoiesis. It is indirectly involved in hematopoiesis because it provides the hematopoietic microenvironment that facilitates hematopoiesis by the parenchymal cells.
Bone marrow: stroma Yellow bone marrow makes up the majority of bone marrow stroma Cells that constitute the bone marrow stroma are: fibroblasts macrophages adipocytes osteoblasts osteoclasts
Bone marrow: barrier Formed by blood vessels of bone marrow Inhibits blood cells from leaving the marrow Only mature blood cells contain the membrane proteins required to attach to and pass the blood vessel endothelium. Hematopoietic stem cells may also cross the bone marrow barrier
Bone marrow: stroma The bone marrow stroma contain mesenchymal stem cells (MSCs) (also called marrow stromal cells). multipotent stem cells that can differentiate into a variety of cell types. To osteoblasts ,chondrocytes , myocytes, adipocytes and beta-pancreatic islets cells. MSCs can also transdifferentiate intoneuronal cells.
Bone marrow: lymphatic role one of the primary lymphoid organs that generate lymphocytes from immature hematopoietic progenitor cells. performs a valve-like function to prevent the backflow of lymphatic fluid in the lymphatic system.
Bone marrow: stem cells 3 types of stem cells: HEMATOPOIETIC STEM CELLS give rise to the three classes of blood cells that are found in the circulation: white blood cells (leukocytes), red blood cells(erythrocytes), and platelets (thrombocytes). MESENCHYMAL STEM CELLS They have the capability to differentiate into osteoblasts,chondrocytes, myocytes, and many other types of cells. They also function as "gatekeeper" cells of the bone marrow. ENDOTHELIAL STEM CELLS
SECONDARY LYMPHOID ORGANS “Peripheral lymphoid organs” Maintain mature naive lymphocytes and initiate an adaptive immune response. sites where lymphocytes are most active (activated by antigen) Provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes.
SECONDARY LYMPHOID ORGANS It is exemplified by the following: lymph nodes Tonsils Peyer's patches Spleen Skin Others that are associated with the mucosa-associated lymphoid tissue (MALT).
LYMPH NODES Also known as lymph glands Small, flattened, oval or bean shaped organs Situated in the course of the collecting lymph vessels. Is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood. The three regions of aggregations of nodes in the body are the groin, armpits and neck
LYMPH NODES Consists of an outer portion called the CORTEX, which contains the lymphoid follicles, and an inner portion called MEDULLA which is surrounded by the cortex on all sides except for a portion known as the HILUM. HILUM is the slight depression on one side of lymph nodes through which the efferent nodal artery enters and a nodal vein leaves the node It is covered by a capsule made of fibrous connective tissue that extends into the node
Lymph nodes Trabeculae – capsular extensions which divide the lymph node internally into a series of compartments that contain lymphatic sinuses or lymphatic tissue
LYMPH NODES Afferent lymph vessels penetrate the capsule and empty into the subcapsular space. The lymph continues thereafter through cortical and medullary sinuses towards the efferent lymph vessels, which emerge from the hilus of the lymph node. The walls of the sinuses can be traversed freely by all components of the lymph, which allows lymphocytes to enter/leave the lymphoid tissue (as part of their constant circulation) or to get in contact with antigens/antigen-presenting cells that may arrive with the lymph.
LYMPH NODES In lymph nodes we find B- and T-lymphocytes, macrophages and reticular cells. Reticular cells form a delicate network between the capsule and trabeculae. The cytoplasm of reticular cells is only weakly eosinophilic. Lymphocytes and macrophages are housed in the network of reticular cells and the reticular fibres formed by them.
LYMPH NODES MEDULLARY CORDS - cord-like extensions of the lymphoid tissue into the medulla. It is where Mature B-lymphocytes (plasma cells) are located PARACORTEX – deep part of the cortex. It is where T-lymphocytes are located
tonsils Masses of lymphoid tissue embedded in mucous membrane There are three groups of tonsils: Palatine tonsils Pharyngeal tonsils Lingual tonsils Tonsillar crypts Epithelium Capsule
Tonsils: palatine Palatine tonsils Occasionally called the faucial tonsils Located on the tonsillar fossae between the pharyngopalatine and glossopalatine arches on each side of the posterior opening of the oral cavity Commonly removed in a tonsillectomy tend to reach their largest size near puberty, and they gradually undergo atrophy thereafter. can become enlarged (adenotonsillar hyperplasia) or inflamed (tonsillitis) and may be surgically removed in tonsillectomy.
Tonsils: palatine Has an extensive system of crypts, which result in a large internal surface. Tonsillar crypts are long and branched Incompletely encapsulated Covered by non-keratinized stratified squamous epithelium that extends into deep and partly branched crypts
Tonsils: palatine Tonsillar crypts greatly increase the contact surface between environmental influences and lymphoid tissue not merely a group of invaginations of the tonsillar epithelium Macrophages and other white blood cells concentrate by the tonsillar crypts in response to the microorganisms attracted to the crypts. provide such an inviting environment to bacteria that may form solidified "plugs" or "stones" within the crypts.
Tonsils: palatine Tonsillar crypts Chronic sinusitis or post-nasal drip Tonsillolith also known as a tonsil stones clusters of calcified material that forms in the crevasses of the tonsils Occur mostly in palatine tonsils but may also occor in lingual tonsils have a foul smell and can contribute to bad breath can obstruct the normal flow of pus from the crypts, and may irritate the throat main substance is mostly collagen, but they have a strong unpleasant odor because of hydrogen sulfide and methyl mercaptan and other substances.
Tonsils: pharyngeal Also known as adenoids, tonsilla pharyngea, Luschka's tonsil, nasopharyngeal tonsil Collection of lymphoid nodules located along the roof and posterior wall of the nasopharynx (upper throat) When it becomes swollen, it may interfere with breathing Removed through adenoidectomy
Tonsils: pharyngeal Covered with ciliated pseudostratified columnar ciliated epithelium Incompletely encapsulated No crypts
Tonsils: pharyngeal Enlarged adenoids, or adenoid hypertrophy Pharyngeal tonsils may become nearly the size of a ping pong ball and completely block airflow through the nasal passages Adenoid facies An atypical appearance of the face due to enlarged adenoids Open mouth/mouth breathing, Long elongated face, prominent incisors, Hypoplastic maxilla, Short upper lip, Elevated nostrils, High arched palate,
Tonsils: lingual Rounded masses of lymphatic tissue that cover the posterior region of the tongue at its base. Their lymphatic tissue are dense and nodular Covered with non-keratinized stratified squamous epithelium Partially surrounded by connective tissue placing them in the group of Partially-Encapsulated Lymphatic Organs They have associated mucous glands which are drained by ducts directly into the single tonsillar crypt. Tonsillar crypts are long and unbranched
Waldeyer‘stonsillar ring Pharyngeal lymphoid ring An anatomical term describing the lymphoid tissue ring located in the pharynx and to the back of the oral cavity.
Peyer’s patches Also known as aggregated lymphatic follicles Found in the wall of small intestine more numerous in younger individuals and become less prominent with age. appear as elongated thickened areas lacking the villi that are typical of intestinal membrane Their macrophages destroy bacteria that are common in the small intestines and prevent them from infecting and penetrating the walls of the intestine
Peyer’s patches The Peyer's patches contain high concentrations of white blood cells (or lymphocytes) that help protect the body from infection and disease. Detect antigens such as bacteria and toxins and mobilize highly specialized white blood cells termed B-cells to produce protein structures called antibodies that are designed to attack foreign entities.
mucosa-associated lymphoid tissue (MALT) diffusion system of small concentrations of lymphoid tissue found in various sites of the body, such as the gastrointestinal tract, thyroid, breast, lung, salivary glands, eye, and skin. populated by lymphocytes such as T cells and B cells, as well as plasma cells and macrophages
SKIN A mucosa-associated lymphoid tissue (MALT) star-shaped cells found mainly in the spiny layer an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense Langerhans cells in the skin are part of the adaptive immune system
Skin: langerhans cells In skin infections, the local Langerhans cells take up and process microbial antigens to become fully functional antigen-presenting cells
Antigen a.k.a. Immunogens These are any substances that stimulate an immune response (antibody production or activation of T cells) and reacts only with the product of response (antibody or activated T cells)
T dependent antigen – antigens that requires T cell’s help to stimulate B cells for antibody production
T independent antigen – antigens with large repetitive structures
Hapten – incomplete immunogen; cannot initiate immune response but can be recognized by antibodies
Superantigen – capable of stimulating up to 20% of the total T cells of an individual
Hypersensitivity (Response) a.k.a. Allergy A condition of exaggerated or inappropriate reaction of the immune response, that are harmful to the host First contact – sensitization Second contact – allergic reaction
Types of Hypersensitivity (Cont.) Type II – Cytotoxic Hypersensitivity Involves bonding of antibodies to cell surface, activation of complement and cell damage Type III – Immune Complex Hypersensitivity Affects the joints, kidneys, blood vessels, and skin Arthus reaction (4-10 hrs) – localized immune complex deposit Systemic immune complex disease – generalized reaction
Types of Hypersensitivity (Cont.) Type IV – Cell-Mediated Hypersensitivity Reaction T cell mediated immune response Allergic contact dermatitis Caused by haptens Erythmia, itching, eczema, or necrosis of skin Transplant rejection
Cells at the Lymphatic System T Cells and B Cells
Lymphocytes Lymphocyte is one of the the five kinds of leukocytes circulating in the blood There are many types of lymphocytes but the most common are: B Lymphocytes T Lymphocytes
These cells are not uniformly distributed in the lymphoid system
They occupy special regions in non-thymic lymphoid structures
When stimulated by antigens, these cells produce large basophilic lymphoid cells called immunoblasts,which contains large numbers of polyribosomes. These cells go through several mitotic cycles and may differentiate in several ways.
B Lymphocytes Also known as B Cells B Cells are the most abundant type of lymphocytes representing 65% of the circulating lymphocytes They are derived from the bone marrow and migrate to non-thymic lymphoid structures
Differentiated B Cells When B Cells migrate to the non-thymic tissue, they nest, proliferate when activated, and differentiate into antibody-secreting plasma cells Some B Cells do not differentiate when activated. Instead, they generate the B memory cells
T Lymphocytes Also known as T Cells They represent 35% of the circulating lymphocytes in the body T Cells precursor are also produced in the bone marrow but they leave the bone marrow and migrate at the thymus
Differentiated T Cells T Cells proliferate at the thymus gland and then migrate to the non-thymic tissues where they mature. They are differentiated into: Helper T cells – stimulates the differentiation of B cells into plasma cells Suppressor T cells – regulates both cellular and humoral immunity and inhibit the action of helper and killer cells
Differentiated T Cells (Cont.) Killer T cells – (a.k.a. cytotoxic lymphocytes or graft-rejecting cells) produce the protein perforins that open holes in foreign cell membranes with consequent cell lysis Cell lysis is a process in which a cell is broken down or destroyed as a result of some external force or condition Memory T cells – react rapidly to the reintroduction of antigen and stimulate production of killer T cells
Differentiated T Cells (Cont.) Helper and suppressor cells are also known collectively as regulator cells. Helper T cells are killed by the retrovirus that causes the disease AIDS. Their immune system are made susceptible to opportunistic infections. Usually these are microorganisms that do not infect healthy individuals.
are a subset of cytokines that are produced by a type of immune cell known as a lymphocyte.
They are protein mediators to direct the immune system response by signalling between its cells.
released by T cells that have been activated by antigens.
Powerful chemical substances secreted by lymphocytes. These soluble molecules help direct and regulate the immune responses.
Lymphokines (cont.) Functions: function in the immune response through stimulating the production of non sensitized lymphocytes and activating macrophages. Chemical messengers that carry messages between the cells of the immune system involved in cell-mediated immune responses that play a part in the body's defense system.
Lymphokines (cont.) Important Lymphokines Involved with T Cells IL-1 (Interleukin-1): has a variety of effects on various types of cells. It acts as a growth regulator of T-cells and it induces other cells to produce proteins relevant to host defense. IL-1 forms a chemotactic gradient for neutrophils and serves as an endogenous pyrogen which produces fever. IL-2 (Interleukin-2): stimulates the proliferation of T-cells. IL-4 (Interleukin-4): causes B cell proliferation and enhanced antibody synthesis. IL-6 (Interleukin-6): has effects on T cell activation, growth, and differentiation. Probably has a major role in the mediation of the inflammatory and immune responses initiated by infection or injury. IL-8 (Interleukin-8): chemotactic attractant for neutrophils. Interferons: Gamma-Interferon (gamma IFN). It is sometimes called "immune interferon" Gamma-interferon has several antiviral effects including inhibition of viral protein synthesis in infected cells. It stimulates IL-2, and antibody production. Lymphotoxins: (Tumor Necrosis Factor-Beta): (TNF-beta is produced by T cells; TNF-alpha is produced by T cells, as well as other types of cells.) TNF kills cells, including tumor cells (at a distance). Colony Stimulating Factor (CSF): several, including GMCSF, cause phagocytic white cells of all types to differentiate and divide.
Interleukines (cont.) Functions: induce growth and differentiation of lymphocytes and hematopoietic stem cells. interact to control the immune system of cells act as messengers between leukocytes involved in either immunologic or inflammatory response
Large Lymphocyte 9-15 microns Oval/bulging nucleus, somewhat looser chromatin structure than the small lymphocyte. No nucleoles Plentiful colorless to light blue cytoplasm Possible isolated bright violet granules
Large Lymphocyte (cont.) Description: Large lymphocytes are the minor part of lymphocytes in the bloodstream. The cell has a large roundish and regular nucleus. Size: 10-15 µm. Cytoplasm: Fairly abundant and sky-blue color; there may be up to 10 azurophilic granules. They are approximately 0.5 µ in diameter. Nucleus: Slightly eccentric located, round or oval, sometimes slightly indented; chromatin is arranged in compact blocks, separated by lighter zones without sharp demarcation, often there are unclearly divided clumps. Nucleoli: Not visible.
displays foreign antigen complexes with major histocompatibility complex (MHC) on their surfaces.
T-cells may recognize these complexes using their T-cell receptors (TCRs).
These cells process antigens and present them to T-cells.
T cells can only 'see' antigen that has been processed and presented by cells via an MHC molecule.
T cells can only 'see' antigen that has been processed and presented by cells via an MHC molecule. Most cells in the body can present antigen to T cells via MHC class I molecules and, thus, act as "APCs"; however, the term is often limited to those specialized cells that can prime T cells (i.e., activate a T cell that has not been exposed to antigen, termed a naive T cell). These cells, in general, express MHC class II as well as MHC class I molecules, and can stimulate "helper"cells as well as "cytotoxic" T cells, respectively.
Antigen-Presenting Cell Macrophage B Cells Dendritic Cells
Macrophages “histiocyte” Largest cells in our immune system Location: connective tissue and lymphoid organs First line of defense against infection Processes and presents antigen to lymphocytes to produce antibodies Arise from monocytes which develops from bone marrow Extend and retract in their locomotion Have many short microvilli and thin undulating folds of their surface, called “lamellipodia” Selective in the kind of particles that they ingest
Functions: Act as a reservoir from plasma and other substances including cells that leaked from the vascular system Transport and carry lymph fluid back from the tissues to the circulatory system. It is the garbage collector, the internal vacuum cleaner sucking up metabolic garbage, toxins and excess fluid from the extracellular fluid of every organ. *Note: Without functioning lymph vessels, lymph cannot be effectively drained and edema typically results.
General Structure As they proceed forward and in their course are joined by other capillaries, they grow larger and first take on an adventitia, and then smooth muscles. Lymphatic system is not closed and has no central pump. Lymph movement occurs despite low pressure due to peristalsis (propulsion of the lymph due to alternate contraction and relaxation of smooth muscle), valves, and compression during contraction of adjacent skeletal muscle and arterial propulsion.
-a type of agranular white blood cell in the vertebrate immune system. -originate from stem cells in the bone marrow. Three major types of lymphocyte 1. Thymus (T) cells - involved in cell-mediated immunity . 2. Bone (B) cells - responsible for humoral immunity or antibody-mediated. 3. Natural Killer (NK) cells - play a major role in defending the host from both tumors and virally infected cells.
Begins with lymph capillaries which are blind-ended, highly permeable and superficial.
Formed by endothelial cells with button-like junctions between them that allow fluid to pass through them when the interstitial pressure is sufficiently high. These button-like junctions consist of protein filaments.
Semilunar valves prevent back-flow of lymph along the lumen of the vessel.
A valve system that prevents the absorbed lymph from leaking. Lymph capillaries have many interconnections or anastomoses between them and form a very fine network.
If tissue fluid builds up the tissue will swell; this is called edema.
As the circular path through the body's system continues, the fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); both ducts drain into the circulatory system at the right and left subclavian veins. The system collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. This is known as a secondary circulatory system.
The lymph capillaries drain the lymph to collecting lymphatics.
These are larger contractile lymphatics, which have valves as well as smooth muscle walls.
As the collecting lymph vessel accumulates lymph from more and more lymph capillaries in its course, it becomes larger and is called the afferent lymph vessel as it enters a lymph node.
Here the lymph percolates through the lymph node tissue and is removed by the efferent lymph vessel. An efferent lymph vessel may directly drain into one of the (right or thoracic) lymph ducts, or may empty into another lymph node as its afferent lymph vessel. Both the lymph ducts return the lymph to the blood stream by emptying into the subclavian veins.
The functional unit of a lymph vessel is known as a lymphangion, which is the segment between two valves., It can act either like a contractile chamber propelling the fluid ahead, or as a resistance vessel tending to stop the lymph in its place.
DIFFUSE LYMPHATIC TISSUE AND LYMPHATIC NODULE
Diffuse Lymphatic Tissue The alimentary canal, respiratory passages and genitourinary tract are guarded by accumulations of diffuse lymphatic tissue, which are not encapsulated. Cells of this tissue are found in the lamina propria of these tracts. They are strategically placed to intercept the entry of antigens, travel to regional lymph nodes, and undergo proliferation and differentiation, with effector lymphocytes, plasma cells and memory cells returning. Large numbers of plasma cells and eosinophils can be found in the lamina propria of the GIT. The diffuse lymphatic tissues and their aggregations of the alimentary canal form the gut associated lymphatic tissue (GALT).
Features of a lymphatic nodule include:1. A germinal centre. This region develops when a lymphocyte has recognised an antigen, and is an indicator of lymphatic tissue response to antigen. It stains less intensely than;2. The marginal zone containing smaller lymphocytes.
Spleen largest accumulation of lymphoid tissue in the body The only one that interposed in the blood circulation Important defense against antigens and site of destruction of aged erythrocytes Important blood filter and antibody forming organ(activated lymphocytes) Composed of reticular tissues with reticular cells, macrophages and APC’s.
General structure Capsule-dense CT that surrounds the spleen. Trabeculae- emerge from the capsule that divides parenchyma or splenic pulp -Large trabeculae originates from the hilum. -carries nerves and arteries into the splenic pulp as well as veins that bring blood back to circulation Hilum- medial surface of the spleen. -where lymphatic vessels leave.
Splenic Pulp Has 2 component: White pulp- consists of periarterial lymphatic sheath and lymphoid nodules Red pulp- splenic cords( Billroth’s cords) and blood Sinusoids.
White Pulp Trabecular arteries follows the course of the connective tissue trabeculae. Periarterial lymphatic sheaths (PALS) – Sheaths of T-lymphocytes that envelopes the trabecular arteries. Central arteries or White Pulp Arteries PALS receive large collection of Lymphocytes (B cells) forming Lymphoid nodules. ( arterioles-occupies Eccentric Position) Marginal Zone- surrounds the lymphoid nodules -consists of many blood sinuses and loose lymphoid tissue. - few lymphocytes but many macrophages found here.
-contains abundant of blood antigens, thus plays a major role in the immunological activities of spleen. After leaving the white pulp, the PALS slowly thins and the central artery/arteriole subdivided to form Penicillar Arteries. - outside diameter of approximately 24um. - Near their termination, surrounded by a thick sheath of reticular cells, lymphoid cells and macrophage.
Red Pulp Composed of: -Splenic Cord -Sinusoid Splenic Cord- contains network of reticular cells supported by reticular fibers. -contains B lymphocytes, macrophages, plasma cells and many blood cells (erythrocytes, platelet and granulocytes) -separated by irregularly shaped white sinusoids. Sinusoid- lined by endothelial cells with a long axes parallel to the long axes of sinusoid. Endothelial cells, developed in reticular fibers set primarily in transverse direction like a hoop on a barrel. - an incomplete basal lamina. -lumen of the red pulp may be very narrow.
Closed circulation- the capillary opens directly into the sinusoid. - blood always remain inside the vessels. Open circulation- prolongation of penicillar arteries open into the splenic cords, and the blood passes through the space between the cells to reach the sinusoid. From the sinusoid, blood proceeds to the red pulp veins that joint together and enter the trabeculae forming trabecular vein. Trabecular Vein- splenic vein originates and emerge from the hilum. - do not have individual muscle walls. - channels hollowed out in the trabecular CT and lined by endothelium.
Function of Spleen Filter, phagocytose, and mount immunological response against blood-borne antigens. Important production site of lymphocytes . (White Pulp) Macrophages in the splenic cords engulf and digest the erythrocytes. The hemoglobin that they contain is broken down to several parts. The protein,globin, is hydrolyzed to amino acids that are reused in protein synthesis. Iron is released from heme and joined transferrin,is transported in the blood to the bone marrow, where it reused in erythropoiesis. Iron-free heme is metabolized to bilirubin, which is excreted by the liver cells.