I. What is the lymphoid system?

A. Offers a pathway for liquid that escapes thecardiovascular system to be returned to it.

You are watching: Lymphoid tissue is mainly reticular connective tissue.

B. Comprises an tissue/organ/vascular systemthat protects the body internal environment from the onslaught of foreignsubstances (i.e. Bacteria, viruses, etc.), likewise from aberrant structures in thebody that develop from ones own tissues (i.e. Malignant growths).

II. The lymphoid system consists of 2major components,

A. The lymph vascular network the permeates every tissues of the body and hastwo functions,

1. Acts come return extracellular fluids to bloodcirculatory system.

2. Acts together pathway for particular cells of theimmune system to move between different parts of body, and also re-enter thecirculatory system. Or move from circulatory system ago to lymphatic organs.

B. The team of organs and also non-organlymphatic tissue associated with the lymph vascular network that likewise serve at least 2 roles.

1. The organs are the lymph nodes, the spleen,the thymus, the appendix, and also the tonsils. The non-organ lymphatic organization wouldbe diffuse lymphatic tissue and also lymph nodules that may be discovered in miscellaneous tissuesof the human body (e.g., Peyer’s job in the wall surface of the ileum).

2. This group of organs and also non-organ lymphatictissue offer at least two roles:

a. Acts to filter cellular and particulate debriout the extracellular fluids.

b. Offers sites of residence, proliferationand activation because that cells of the immune system.

III. Tissues of lymphoid guts consistof two significant cell types,

A. fixed cells - usually reticulocytes, but also folliculardendritic cells

1. reticulocytes - connective tissue cells that may secrete a finematrix the reticular fibers the these cells prolong cytoplasmic processesthrough and also around.

2. follicular dendritic cells (FDCs) – appear similar to reticulocytes in shape, butare yes, really a form of macrophage - uncovered in the germinal centers the lymph nodes.These cells bind international antigens and also interact through lymphocytes together antigenpresenting cells.

B. free cells

1. Macrophages

2. Various classes of lymphocytes (B and T)

IV. There are 2 significant types that lymphoidtissue,

A. Non-nodular lymphoid tissue

1. Unencapsulated, sub-epithelial, aggregationsof lymphocytes that deserve to occur anywhere in the body

2. Non-nodular parenchyma that lymphoid organssuch together the thymus, lymph nodes and also spleen

B. Nodular lymphoid tissue - sphericallyarranged aggregations the lymphocytes that are dubbed lymphatic nodulesconsisting that a cortex and also germinal center.

1. Nonencapsulated lymph nodules

2. Encapsulated lymph nodes

3. Spleenic white pulp

4. MALT (mucosa-associated lymphoid tissue) – nodular aggregations oflymphoid tissue that are connected with the lining that the cradle tract,e.g., tonsils, appendix, Peyer’s patches,. These tissues make up the largest(admittedly diffuse) lymphoid body organ in the body and also contain about 70% of thebody’s immune cells.

V. Immune system attributes of thelymphoid system

A. There are 4 significant cell types involved in animmune response.

1. macrophages - procedure antigens and also present them to both B-and T- lymphocytes, for this reason causing the activation of these cells in humoral andcell-mediated immune responses.

2. Committed macrophage antigen presentingcells such as dendritic cellsand Langerhan’s cells.

2. B- lymphocytes - responsible because that antibody production. Humoralresponse come an antigen that results in the secretion of antitoxin by plasmacells acquired from B-lymphocytes.

3. T-lymphocytes - responsible because that cell-mediated immuneresponse. Additionally involved in mediating the activation the B-lymphocytes come produceantibodies.

B. Immune response:

1. Entails the acknowledgment of foreign antigensby specific lymphatic cells

a. antigen - a molecule that has actually in its make-up features that willcause the activation of details cells in the immune system.

b. Antigenic attributes - components of themolecules structure that the cell of the immune device will recognize asforeign and will react to.

c. The portions of the antigen that immunesystem cell specifically recognize are dubbed epitopes.

d. epitopes are typically parts the protein molecule that type part the thecell membrane or wall of organisms that invade the bodies tissues.

2. One immune an answer results in changes in themetabolism and chemical programming that lymphocytes that allows them come act indestroying the foreign substance or organism

C. One immune solution may be either humoral or cell-mediated.

1. humoral immune response - an answer of caused B-lymphocytes that havebeen presented v a international antigen.

a. Mediated by interaction in between macrophage,T-lymphocyte and B-lymphocyte

a. Reasons clonal proliferation that activatedB-lymphocytes

b. Clonally created B-lymphocytes differentiateinto plasma cells or memory B-lymphocytes

* plasma cell are brief lived and secrete copiousamounts the antibody that are particular for one epitope that the antigen

** antibodies act come identify foreign cells because that attackby other components of the immune device such as T-lymphocytes

** antitoxin can also identify particulates andviruses because that phagocytosis and also destruction by other leucocytes such asneutrophils and eosinophils.

* B-lymphocyte storage cells stay dormant andwill swiftly respond come future encounters v the very same antigen by clonalproliferation causing the formation of plasma cells the secreteantibodies details for that antigen.

2. cell-mediated immune response - antigen activates T-lymphocytes to producecytotoxic substances that cause the damage of the antigen containing cell

a. Mediated through interaction in between macrophageand T-helper (Th) and T-cytotoxic (Tc) lymphocytes

b. Reasons clonal proliferation the Tc lymphocytes

c. Clonally produced Tc lymphocytes identify into one of two people Tc memory cells or Tc effector cells

* Tc effectorcells - actively kill invading international cells

* Tc memorycells continue to be dormant and will promptly respond come future invasions by foreigncells express the exact same antigen

D. There is no the activities of cells of thelymphoid system, her life on earth after birth would be quick indeed. Thisfact is evident in babies that space born lacking a functional immune system.Unless extraordinary measures, together as total sterile isolation, are taken theydie soon after birth. Another, perhaps more relevant instance is AcquiredImmune Deficiency Syndrome or AIDS in which human being T-lymphatic virus kind III(HTLVIII) death the T4-helper sub-set the T-lymphocytes. This outcomes in thebody no being able to efficiently and effectively cope through infectiousorganisms via a cell-mediated immune response. Without treatment, the patientwill soon succumb come a mortal disease.

VI. Structure and role of lymphoidorgans

A. Complimentary lymphatic nodules (lymphatic follicles).

1. Mainly found in the lamina propria the the cradle tract, the respiratory tract,and urinary passages. Might “appear” or “disappear” relying on the immunesituation. Nodules in tonsils and Peyer’s patches space persistent.

a. Lamina propria - layer of loosened connective tissue thatunderlies the epithelia lining the lumens of internal organs.

2. No connective tissue capsule surrounds freelymphatic nodules

3. The resolved and totally free cells the a lymphaticnodule space organized right into a densely pack and much more or less spherical structure.When the nodule is energetic in an immune response, it has a cortex and a much more orless central germinal facility (a medullary structure).

4. Lymphoid tissue is composed of:

a. Fixed cells – reticulocytes in cortex and folliculardendritic cells in germinal center.

b. Free cells - B-lymphocytes, immunoblasts (clonallydividing B-lymphocytes), plasma cells, macrophages, A few eosinophils and mastcells.

* mostly small lymphocytes with basophilicnuclei comprise heterochromatin in the characteristic “clock-face” organization.

* cell in the nodule are mainly B-lymphocytes.

* as soon as a germinal center (medullary comnponent)is present, the is regularly less densely stained 보다 the cortical areas of thenodule.

* in a germinal center, numerous of the medullarycells are activated B-lymphocytes experience clonal departments - most of theresulting cells will certainly mature into plasma cells

5. Development of lymphatic nodules is dependenton stimulation by international antigens - newborn animals and animals reared inaseptic atmospheres have very few lymphatic nodules.

6. Tonsils and Peyer’s patches room characterizedas groups totally free lymphatic nodules that persist transparent life.

B. Lymph nodes

1. Current along the course of lymphaticvessels. Many lymph vessels may attach to a lymph node.

2. Oval or bean shaped bodies surrounding by adense connective organization capsule

3. Septa or trabeculae prolong from capsule intolymph node.

4. Filled with stroma consists of reticularfibers and also cells.

5. Stroma offers a assistance network for largenumbers of lymphocytes.

6. Node is composed of:

a. A dense outer cortex that consists of numerouslymphatic nodules. Countless lymphocytes, macrophages, various other antigen presentingcells (APCs), plasma cells and reticulocytes are present. Follicular dendriticcells are found in the germinal centers that lymph nodules that are in thecortex.

b. A less thick medulla consisting oflymphocytes arranged in strands referred to as medullary cords.

* sinuses are present in the medulla

* cords and also sinuses extend toward a centralhilus the is essentially a large trabecula projecting right into the lymph node fromthe connective tissue capsule.

** arteries enter and veins and lymph vesselsexit through the hilus

** blood ship branch native the hilus right into thecortical an ar where they offer rise come "bulbs" that capillaries withinthe germinal centers the the cortical lymph nodules

7. In between the cortex and medulla is theparacortical region or thymic dependent zone that the node that includes denselypacked cells that are mostly T-lymphocytes.

a. This region lacks lymphocytes in pets thathave had actually the thymus gotten rid of at birth.

8. Cells exterior the paracortical region aremostly B-lymphocytes.


C. Tonsils - 3 types that are characterized by your structure and also their locationin mouth and pharynx.

1. Palatine tonsils

a. ~ above left and also right in rear area of oral cavity.

b. Dense lymphoid tissue that forms a band oflymphatic nodules the lie justbelow a non-keratinized, stratified, squamous epithelium lining the dental cavityin this region.

c. Overlying epithelium creates invaginationscalled multiple crypts thatpenetrate right into the band of nodules.

d. These crypts act together collecting locations forcellular debris and also bacteria and some living lymphocytes the havemigrated into the crypts.

e. The tape of lymph nodules is separated fromunderlying tissues by a partial capsule of dense connective tissue.


2. Pharyngeal tonsils

a. Diffuse lymphoid tissue containing nodules,but no crypts.

b. Greatly lie in ~ a typical pseudostratifiedciliated columnar respiratory tract epithelium in rear roof of pharynx. Some areas of the spanning epithelium perhaps stratified squamous.

c. A slim partial capsule of dense connectivetissue separates the lymphoid tissue from underlying tissue.

3. Lingual tonsils

a. Situated in the source of tongue.

b. Every lingual tonsil consists of numerous.lymphoid nodules bordering a solitary crypt

c. The crypt is inside wall by a non-keratinized, stratified, squamous epithelium.

d. A slim partial capsule of thick connectivetissue separates the lymphoid organization from basic tissue.

D. Thymus

1. Ar - situated above the heart where thegreat ship connect.

2. Prominence - during early life as soon as the cellularmediated ingredient of the immune mechanism develops. Experience atrophy in laterlife, in ~ which time it loses its functional significance.

3. The thymus consists of multiple lobes eachcontaining characteristics cortical and also medullary structure; however, this arenot lymphatic nodules (i.e., not a spherical structure that is unique fromsurrounding cells). A connective tissue capsule surrounds the thymus.


4. Embryologically, the thymus has actually a dualorigin.

a. Epithelial organization of embryo’s pharynx areinternalized during development and move to site of thymus dorsal come heart.

b. These epithelial organization are attacked bylymphoblasts (immature T-lymphocytes) the originate from stem cell in thebone marrow.

c. The invading cell organize themselves into thecortical and medullary parts of lobules.

* Cortical area is composed of dense population ofso-called thymocytes, that space T-lymphocytes, and also scattered epithelialreticular cell that have multiple processes and partially compartmentalize thethymocytes.

* this cells surround a main zone that looselymphatic tissue dubbed the medullary region that is composed of fewer thymocytesand more epithelial reticular cells.

* The cortical and medullary zones of lobulesare all consistent with each other.

d. Various other cell varieties found in the thymusare:

* macrophages

* plasma cells

* mast cells

5. Cortical class of thymus

a. Site of lymphocyte manufacturing - divisions oflymphoblast cells.

b. Thus, there is significant mitotic activityof lymphoblasts

c. Epithelial reticular cells are much less numerousin this area and also have thin and long procedures that evelope groups of developingthymocytes. No reticular fibers space present.


d. This sheath that epithelial reticular cells, aswell together the desmosomes that connect adjacent reticular cells and also the thickbasement membrane the underlies castle act to separate arising thymocytesfrom the circulatory system.

6. Blood-thymus obstacle - only existing in thecortex, acts come prevent most blood born international antigens from reachingdeveloping thymocytes. This barrier consists of:

a. Non-fenestrated, constant endothelium ofblood capillaries

b. Pericytes and epithelial-reticular cell thatform a sleeve approximately the capillaries in addition to neighboring connectivetissue.

c. Macrophages the are current in theconnective tissue.

d. Thick basal lamina that epithelial reticularcells

e. Desmosome connections creating tight junctionsbetween nearby endothelial cells of capillaries, as well as similarconnections in between surrounding epithelial-reticular cells

7. Medullary zone

a. Has mostly epithelial-reticular cellsand under T-lymphoblasts and also lymphocytes 보다 the cortex.

b. Likewise contains devoted structures known asHassall"s corpuscles -function unknown

* covers a central, eosinophilic, hyalinecore surrounding by concentric layers of epithelial reticular cells containing lotsof keratin.

*sometimes they are calcified.

*these structures room characteristic that thymus.

8. Blood supply

a. Branches from the inner thoracic andinferior thyroid arteries that permeate the capsule bordering the thymus

b. Expand into thymus along interlobular septa.

c. Capillaries branch into the cortico-medullaryjunction area and also extend into the cortex. Blood-thymus obstacle is current asdescribed above.

d. These eventually arch right into the medulla wherethey drain into venules

* In the medulla, the sheath cells of theblood-thymus obstacle are lost and vessels become permeable.

*cells may move right into or out of the thymus inthis region by entering the circulatory system.

e. Venules connect to veins that exit thymusalong connective tissue septa.

f. Cells entering thymus indigenous bone marrow arrivevia the circulatory system and also enter in the medullary zone native which lock canmove to the cortical areas.

g. There are no afferent lymphatic vessels inthymus. So the does not act as filter for lymphatic fluids.

h. Just a few efferent lymphatic vessels arepresent, and also these are associated with the blood vessels.

9. Physiology the thymus loved one to immunesystem.

a. Recall the T-lymphocytes space responsible forcell-mediated immune responses and likewise for communicating with B-lymphocytes tocause those cells to produce antibodies against particular antigens.

b. The thymus have the right to be taken into consideration a proliferationand maturation center for T-lymphocytes. Precursor cells move from bonemarrow come thymus entering the body organ via blood vessels in medullary zone. Thesecells experience mitosis and maturation in cortical zone and also then leave thymusthrough blood vessels of medullary zone to go around their miscellaneous activities.This function for the thymus is greatly completed soon after puberty in ~ which timethe body organ undergoes “involution.” together this occurs, the thymus accumulation aconsiderable lot of unilocular fat. Little areas of energetic lymphoid tissueare retained and also will continue to produce little numbers of tires T-lymphocytesfor the remainder the a who life; however, this task is supposedly not anecessity since the thymus of one adult can be removed without leading to immunesystem problems.

E. Spleen

1. General characteristics

a. Largest piece the lymphatic tissue in body.

b. Website of formation of set off lymphocytesthat enter the circulatory system.

c. Likewise important in recycling of materials ofworn-out blood cells.

d. Can be claimed to act together filter the blood both inan immunologic feeling (that is that mediates materials of immune response), butalso in the feeling of removing worn the end erythrocytes from circulation.

2. Structure

a. Surrounding by a dense connective tissuecapsule that extends procedures (trabeculae) into lymphatic organization of thisorgan.

* connective tissue includes nerves, bloodvessels,lymph vessels, and also smooth muscle.

* A hilus the connective organization is presentmedially.

* Blood vessels and nerves run with the hilumand enter the spleenic pulp via the trabeculae. There room no lymph vessels inthe pulp.

* Pulp is divided into lymphatic nodules ofwhite pulp, surrounded by a spongy lymphatic tissue called red pulp. Colordesignations have to do through appearance in freshly cut open organ.


b. Red pulp

* forms spongy reticular tissue that is composedof cords (cords of Bilroth) of cells the surround blood sinusoids (cavities).

* sustained by a network of reticular cells andtheir connected fibers.

* Cell species present - macrophages, monocytes,lymphocytes, plasma cells, and also various blood cell (i.e. Granulocytes anderythrocytes).

* Blood sinusoids are present - sites of cellularexchange in between spleen and circulatory system. Cell can go into or leaving spleenthrough large spaces in between endothelial cells lining sinusoids.

c. White pulp

*Concentrations that lymphatic tissue within thered pulp the surrounds sections of central arteries creating nodules alongtheir lengths. Note that the “central” arteries frequently do not run with thecenter of the lymph nodule.

*Nodules consist of reticular mesh v spacesin mesh gift filled through lymphocytes and macrophages. Central artery is identifyingcharacteristic.

*Marginal zone surrounding white pulp nodules -contains few lymphocytes, however many proactively phagocytic dendritic cell (a typeof macrophage) v branching processes.

*Marginal region acts together a filter to pull foreignantigens the end of blood so that lymphocytes deserve to be activated to take part in animmune response.

* Both B- and also T-lymphocytes in white pulp. MainlyT-lymphocytes in periarterial lymphatic sheath (PALS) sheath surroundingcentral artery, B-lymphocytes in lymphatic tissue of white pulp surroundingsheath (i.e. Peripheral white pulp). The B-lymphocytes and also associated cell areorganized into typical lymphatic nodules.

* various other cell types present - monocytes, plasmacells, various other macrophages.


3. Blood circulation in spleen

a. Arteries get in pulp via trabeculae.

b. Branches that arteries expand into white pulpforming the central arteries the the white pulp.

c. This arteries room surrounded by a sheath oflymphocytes mainly T-lymphocytes) that form the periarterial lymphatic sheath(PALS) prolonging along the length of the artery.

d. Branches of main artery expand into whitepulp.

e. Few of these leaving white pulp and also then loopback towards it emptying into sinusoids that kind part of marginal zone the looselymphoid organization that surrounds white pulp.

f. Various other branches of main arteries extendinto red pulp to type the pulp arteries the empty into various sinusoids ofthis tissue.

g. Blood native the sinusoids is accumulated intored pulp veins. These trabecular veins integrate to form the spleenic vein thatleaves the spleen through the hilus.

h. Blood circulation is more than likely both open(blood empties into tissue of pulp that is percolates through and is latercollected right into sinusoids that connect to venules) and also closed (blood native arteriolesempties into sinusoids that room directly associated to venules).

4. Functions of the spleen

a. Manufacturing of blood cells

* In embryo, erythrocytes, neutrophils,basophils, and also eosinophils are produced in spleen. This stops about the time ofbirth.

* In adult, B-lymphocytes and also T-lymphocytesactivated in spleen and can get in circulatory system.

b. Devastation of erythrocytes

* Worn the end erythrocytes are phagocytosed anddigested

* Hemoglobin is broken down right into bilirubin andferritin.

c. Recycling the ferritin from worn outerythrocytes because that synthesis the hemoglobin

* Bilirubin is returned to the blood and also carriedto the liver whereby it is excreted and also passed the end of body in the bile.

e. Immune response

* website of activation that both T- andB-lymphocytes

* this cell varieties interact through dendritic cells(macrophages) that existing antigen to these lymphocytes.

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f. Storage of erythrocytes that can be releasedinto circulatory system when needed.