I. What is the lymphoid system?

A. Offers a pathway for liquid that escapes thecardiovascular device to be went back to it.

You are watching: The thymus is the only lymphoid organ that does not directly fight antigens.

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

II. The lymphoid system consists of 2major components,

A. The lymph vascular network the permeates all tissues that the body and also hastwo functions,

1. Acts to return extracellular fluids come bloodcirculatory system.

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

B. The group of organs and also non-organlymphatic tissue connected with the lymph vascular network that also serve at the very least 2 roles.

1. The organs room the lymph nodes, the spleen,the thymus, the appendix, and also the tonsils. The non-organ lymphatic tissue wouldbe diffusive lymphatic tissue and lymph nodules that may be discovered in various tissuesof the body (e.g., Peyer’s spot in the wall surface of the ileum).

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

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

b. Provides sites the residence, proliferationand activation for cells that the immune system.

III. Organization of lymphoid guts consistof two major cell types,

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

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

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

B. free cells

1. Macrophages

2. Assorted classes the lymphocytes (B and also T)

IV. There are 2 significant types the lymphoidtissue,

A. Non-nodular lymphoid tissue

1. Unencapsulated, sub-epithelial, aggregationsof lymphocytes that have the right to occur everywhere in the body

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

B. Nodular lymphoid organization - sphericallyarranged aggregations that lymphocytes that are called lymphatic nodulesconsisting that a cortex and germinal center.

1. Nonencapsulated lymph nodules

2. Encapsulated lymph nodes

3. Spleenic white pulp

4. MALT (mucosa-associated lymphoid tissue) – nodular aggregations oflymphoid organization that are associated with the lining the the digestive tract,e.g., tonsils, appendix, Peyer’s patches,. This tissues comprise the largest(admittedly diffuse) lymphoid body organ in the body and also contain around 70% the thebody’s immune cells.

V. Immune system attributes of thelymphoid system

A. There space 4 major cell types involved in animmune response.

1. macrophages - process antigens and present them to both B-and T- lymphocytes, therefore 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 for antibody production. Humoralresponse come an antigen that outcomes in the secretion of antitoxin by plasmacells derived from B-lymphocytes.

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

B. Immune response:

1. Entails the recognition of international antigensby particular lymphatic cells

a. antigen - a molecule that has actually in that is make-up characteristics that willcause the activation of specific cells in the immune system.

b. Antigenic qualities - parts of themolecules framework that the cells of the immune device will recognize asforeign and also will reaction to.

c. The parts of the antigen that immunesystem cells specifically recognize are called epitopes.

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

2. An immune an answer results in transforms in themetabolism and chemical programming of lymphocytes that allows them to act indestroying the foreign substance or organism

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

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

a. Mediated through interaction between macrophage,T-lymphocyte and also B-lymphocyte

a. Reasons clonal proliferation the activatedB-lymphocytes

b. Clonally developed B-lymphocytes differentiateinto plasma cell or memory B-lymphocytes

* plasma cells are short lived and secrete copiousamounts that antibody that are particular for an epitope of the antigen

** antibodies act come identify foreign cells because that attackby other contents of the immune mechanism such together T-lymphocytes

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

* B-lymphocyte storage cells stay dormant andwill quickly respond to future encounters through the same antigen by clonalproliferation resulting in the development of plasma cells that secreteantibodies certain for the antigen.

2. cell-mediated immune response - antigen activates T-lymphocytes come producecytotoxic substances that cause the destruction of the antigen comprise cell

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

b. Causes clonal proliferation the Tc lymphocytes

c. Clonally created Tc lymphocytes identify into either Tc memory cells or Tc effector cells

* Tc effectorcells - proactively kill invading foreign cells

* Tc memorycells stay dormant and will rapidly respond come future intrusions by foreigncells expressing the exact same antigen

D. There is no the tasks of cell of thelymphoid system, your life on planet after birth would certainly be short indeed. Thisfact is noticeable in babies that room born lacking a functional immune system.Unless extraordinary measures, such as full sterile isolation, are taken theydie soon after birth. Another, perhaps an ext relevant example is AcquiredImmune Deficiency Syndrome or AIDS in which human T-lymphatic virus kind III(HTLVIII) death the T4-helper sub-set of T-lymphocytes. This results in thebody no being able to efficiently and also effectively cope with infectiousorganisms via a cell-mediated immune response. Without treatment, the patientwill quickly succumb to a mortal disease.

VI. Structure and duty of lymphoidorgans

A. Cost-free lymphatic nodules (lymphatic follicles).

1. Mainly discovered in the lamina propria of the digestive tract, the respiratory tract,and urinary passages. May “appear” or “disappear” relying on the immunesituation. Nodules in tonsils and Peyer’s patches are persistent.

a. Lamina propria - class of loosened connective tissue thatunderlies the epithelia lining the lumens of interior organs.

2. No connective tissue capsule surrounding freelymphatic nodules

3. The resolved and totally free cells the a lymphaticnodule are organized right into a densely pack and an ext or less spherical structure.When the nodule is active in an immune response, it has a cortex and also a an ext orless main germinal facility (a medullary structure).

4. Lymphoid tissue is composed of:

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

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

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

* cells in the nodule are greatly B-lymphocytes.

* once a germinal facility (medullary comnponent)is present, that is regularly less densely stained 보다 the cortical regions of thenodule.

* in a germinal center, countless of the medullarycells space activated B-lymphocytes experience clonal divisions - 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 settings have very couple of lymphatic nodules.

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

B. Lymph nodes

1. Current along the food of lymphaticvessels. Lot of lymph ship may connect to a lymph node.

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

3. Septa or trabeculae expand from capsule intolymph node.

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

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

6. Node consists of:

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

b. A less thick medulla consists oflymphocytes arranged in strands dubbed medullary cords.

* sinuses are current in the medulla

* cords and also sinuses expand toward a centralhilus that is basically a large trabecula projecting right into the lymph node fromthe connective organization capsule.

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

** blood vessels branch from the hilus into thecortical region where they offer rise to "bulbs" the capillaries withinthe germinal centers the the cortical lymph nodules

7. Between the cortex and medulla is theparacortical region or thymic dependence zone that the node that consists of denselypacked cells the are mainly T-lymphocytes.

a. This region lacks lymphocytes in animals thathave had the thymus removed at birth.

8. Cells outside the paracortical an ar aremostly B-lymphocytes.


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

1. Palatine tonsils

a. Top top left and right in behind area of oral cavity.

b. Dense lymphoid organization that creates a band oflymphatic nodules that lie justbelow a non-keratinized, stratified, squamous epithelium lining the oral cavityin this region.

c. Overlying epithelium creates invaginationscalled lot of crypts thatpenetrate into the tape of nodules.

d. This crypts act as collecting places forcellular debris and bacteria and some life lymphocytes that havemigrated right into the crypts.

e. The band of lymph nodules is be separate fromunderlying organization by a partial capsule of dense connective tissue.


2. Pharyngeal tonsils

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

b. Mainly lie in ~ a common pseudostratifiedciliated columnar respiratory tract epithelium in rear roof of pharynx. Some locations of the spanning epithelium possibly stratified squamous.

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

3. Lingual tonsils

a. Located in the root of tongue.

b. Each lingual tonsil is composed of numerous.lymphoid nodules neighboring a single crypt

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

d. A thin partial capsule of thick connectivetissue the end the lymphoid organization from basic tissue.

D. Thymus

1. Location - situated over the heart wherein thegreat vessels connect.

2. Prominence - throughout early life when the cellularmediated ingredient of the immune device develops. Undergoes atrophy in laterlife, in ~ which time it loser its useful significance.

3. The thymus is composed of lot of lobes eachcontaining characteristic cortical and medullary structure; however, these arenot lymphatic nodules (i.e., no a spherical framework that is distinct fromsurrounding cells). A connective tissue capsule surrounds the thymus.


4. Embryologically, the thymus has a dualorigin.

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

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

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

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

* these cells surround a main zone that looselymphatic tissue referred to as the medullary ar that is composed of under thymocytesand an ext epithelial reticular cells.

* The cortical and also medullary area of lobulesare all constant with every other.

d. Other cell varieties found in the thymusare:

* macrophages

* plasma cells

* mast cells

5. Cortical great of thymus

a. Site of lymphocyte manufacturing - divisions oflymphoblast cells.

b. Thus, over there is substantial mitotic activityof lymphoblasts

c. Epithelial reticular cell are much less numerousin this area and also have thin and long procedures that evelope teams of developingthymocytes. No reticular fibers room present.


d. This sheath the epithelial reticular cells, aswell together the desmosomes that connect surrounding reticular cells and the thickbasement membrane the underlies lock act to separate developing thymocytesfrom the circulatory system.

6. Blood-thymus obstacle - only current 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 around the capillaries in addition to surrounding connectivetissue.

c. Macrophages the are present in theconnective tissue.

d. Special basal lamina of epithelial reticularcells

e. Desmosome connections developing tight junctionsbetween adjacent endothelial cells of capillaries, and similarconnections in between surrounding epithelial-reticular cells

7. Medullary zone

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

b. Also contains dedicated structures recognized asHassall"s corpuscles -function unknown

* consist of a central, eosinophilic, hyalinecore surrounded by concentric layers of epithelial reticular cell containing lotsof keratin.

*sometimes they room calcified.

*these structures are characteristic of thymus.

8. Blood supply

a. Branches indigenous the internal thoracic andinferior thyroid arteries that permeate the capsule neighboring the thymus

b. Extend into thymus follow me interlobular septa.

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

d. These ultimately arch right into the medulla wherethey drainpipe into venules

* In the medulla, the sheath cell of theblood-thymus barrier are lost and vessels come to be permeable.

*cells might move into or out of the thymus inthis an ar by start the circulatory system.

e. Venules affix to veins that leave thymusalong connective tissue septa.

f. Cell 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 room no afferent lymphatic ship inthymus. So the does no act as filter for lymphatic fluids.

h. Only a few efferent lymphatic ship arepresent, and these are linked with the blood vessels.

9. Physiology the thymus relative to immunesystem.

a. Recall that T-lymphocytes are responsible forcell-mediated immune responses and additionally for communicating with B-lymphocytes tocause those cells to create antibodies against particular antigens.

b. The thymus have the right to be taken into consideration a proliferationand maturation center for T-lymphocytes. Precursor cells migrate from bonemarrow to thymus entering the organ via blood ship in medullary zone. Thesecells undergo mitosis and also maturation in cortical zone and then leaving thymusthrough blood ship of medullary zone come go about their assorted activities.This duty for the thymus is mainly completed soon after puberty at 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 will proceed to produce tiny numbers of tires T-lymphocytesfor the remainder of a who life; however, this task is apparently not anecessity because the thymus of one adult can be removed without causing immunesystem problems.

E. Spleen

1. Basic characteristics

a. Biggest piece the lymphatic organization in body.

b. Website of formation of triggered lymphocytesthat go into the circulatory system.

c. Also important in recycling of components ofworn-out blood cells.

d. Have the right to be said to act together filter that blood both inan immunologic sense (that is it mediates components of immune response), butalso in the sense of removed worn out erythrocytes native circulation.

2. Structure

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

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

* A hilus of connective tissue is presentmedially.

* Blood vessels and also nerves run with the hilumand get in the spleenic pulp via the trabeculae. There space no lymph vessels inthe pulp.

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


b. Red pulp

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

* supported by a network that reticular cells andtheir associated fibers.

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

* Blood sinusoids are present - website of cellularexchange in between spleen and circulatory system. Cell can get in or leave spleenthrough large spaces in between endothelial cell lining sinusoids.

c. White pulp

*Concentrations the lymphatic organization within thered pulp the surrounds sections of central arteries developing nodules alongtheir lengths. Note that the “central” arteries regularly do not run v thecenter of the lymph nodule.

*Nodules consists reticular mesh with spacesin mesh gift filled with lymphocytes and macrophages. Central artery is identifyingcharacteristic.

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

*Marginal region acts together a filter to traction foreignantigens out of blood so the lymphocytes have the right to be set off 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 organization of white pulp surroundingsheath (i.e. Peripheral white pulp). The B-lymphocytes and also associated cells areorganized into common lymphatic nodules.

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


3. Blood circulation in spleen

a. Arteries enter pulp via trabeculae.

b. Branches of arteries extend into white pulpforming the main arteries that the white pulp.

c. These arteries room surrounded through a sheath oflymphocytes mostly T-lymphocytes) that type the periarterial lymphatic sheath(PALS) extending along the size of the artery.

d. Branches of central artery extend into whitepulp.

e. Several of these leave white pulp and then loopback towards it emptying into sinusoids that type part the marginal zone of looselymphoid organization that surrounds white pulp.

f. Other branches of central arteries extendinto red pulp to type the pulp arteries the empty right into various sinusoids ofthis tissue.

g. Blood native the sinusoids is accumulated intored pulp veins. This trabecular veins combine to kind the spleenic vein thatleaves the spleen v the hilus.

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

4. Functions of the spleen

a. Manufacturing of blood cells

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

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

b. Damage of erythrocytes

* Worn the end erythrocytes are phagocytosed anddigested

* Hemoglobin is broken down into bilirubin andferritin.

c. Recycling that ferritin indigenous worn outerythrocytes because that synthesis of hemoglobin

* Bilirubin is returned to the blood and also carriedto the liver where it is excreted and passed out of human body in the bile.

e. Immune response

* website of activation that both T- andB-lymphocytes

* this cell types interact with dendritic cells(macrophages) that existing antigen to this lymphocytes.

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f. Warehouse of erythrocytes that can be releasedinto circulatory device when needed.