I. What is the lymphoid system?

A. Gives a pathway for fluid that escapes thecardiovascular mechanism to be returned to it.

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

B. Comprises an tissue/organ/vascular systemthat protects the bodies internal environment from the onslaught of foreignsubstances (i.e. Bacteria, viruses, etc.), also from aberrant frameworks in thebody that construct 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 that the body and hastwo functions,

1. Acts to return extracellular fluids to bloodcirculatory system.

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

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

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

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

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

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

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

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

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

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

B. free cells

1. Macrophages

2. Various classes that lymphocytes (B and T)

IV. There are 2 major types that lymphoidtissue,

A. Non-nodular lymphoid tissue

1. Unencapsulated, sub-epithelial, aggregationsof lymphocytes that can occur all over in the body

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

B. Nodular lymphoid tissue - sphericallyarranged aggregations that lymphocytes that are referred to as 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 linked with the lining of the digestive tract,e.g., tonsils, appendix, Peyer’s patches,. This tissues comprise the largest(admittedly diffuse) lymphoid organ in the body and also contain around 70% the thebody’s immune cells.

V. Immune system attributes of thelymphoid system

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

1. macrophages - process 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. Dedicated macrophage antigen presentingcells such together dendritic cellsand Langerhan’s cells.

2. B- lymphocytes - responsible because that antibody production. Humoralresponse to an antigen that results in the cheap of antitoxin by plasmacells obtained from B-lymphocytes.

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

B. Immune response:

1. Requires the acknowledgment of international antigensby certain lymphatic cells

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

b. Antigenic attributes - components of themolecules structure that the cells of the immune mechanism will recognize asforeign and also will react to.

c. The sections of the antigen that immunesystem cells specifically acknowledge are referred to as epitopes.

d. epitopes are usually parts that protein molecules that form part that thecell membrane or wall of biology that invade the bodies tissues.

2. An immune an answer results in alters in themetabolism and also chemical programming of lymphocytes that enables them come act indestroying the international substance or organism

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

1. humoral immune response - response of caused B-lymphocytes the havebeen presented through a international antigen.

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

a. Causes clonal proliferation the activatedB-lymphocytes

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

* plasma cell are quick lived and also secrete copiousamounts the antibody that are specific for one epitope that the antigen

** antitoxin act come identify foreign cells because that attackby other materials of the immune mechanism such as T-lymphocytes

** antitoxin can likewise identify particulates andviruses for phagocytosis and destruction by various other leucocytes such asneutrophils and also eosinophils.

* B-lymphocyte memory cells stay dormant andwill promptly respond come future encounters through the same antigen by clonalproliferation causing the development of plasma cells that secreteantibodies specific for the antigen.

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

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

b. Causes clonal proliferation the Tc lymphocytes

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

* Tc effectorcells - actively kill invading foreign cells

* Tc memorycells remain dormant and also will rapidly respond to future intrusions by foreigncells to express the exact same antigen

D. There is no the activities of cell of thelymphoid system, your life on earth after birth would certainly be short indeed. Thisfact is apparent in babies that are born doing not have a practical immune system.Unless especially measures, such as total sterile isolation, room taken theydie quickly after birth. Another, perhaps much more relevant example is AcquiredImmune Deficiency Syndrome or AIDS in which person T-lymphatic virus type III(HTLVIII) kills the T4-helper sub-set that T-lymphocytes. This outcomes in thebody not being able come efficiently and effectively cope v infectiousorganisms via a cell-mediated immune response. Without treatment, the patientwill shortly succumb to a mortal disease.

VI. Structure and duty of lymphoidorgans

A. Complimentary lymphatic nodules (lymphatic follicles).

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

a. Lamina propria - great of loose connective organization thatunderlies the epithelia lining the lumens of inner organs.

2. No connective organization capsule surrounds freelymphatic nodules

3. The solved and complimentary cells of a lymphaticnodule room organized into a densely pack and an ext or much less spherical structure.When the nodule is energetic in an immune response, it has a cortex and a more orless central germinal center (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 couple of eosinophils and mastcells.

* mostly little lymphocytes v basophilicnuclei containing heterochromatin in the properties “clock-face” organization.

* cell in the nodule are mainly B-lymphocytes.

* when a germinal facility (medullary comnponent)is present, that is often less densely stained 보다 the cortical areas of thenodule.

* in a germinal center, plenty of of the medullarycells are 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 foreign antigens - child animals and also animals reared inaseptic atmospheres have very couple of lymphatic nodules.

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

B. Lymph nodes

1. Current along the food of lymphaticvessels. Many lymph vessels may affix to a lymph node.

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

3. Septa or trabeculae extend from capsule intolymph node.

4. Filled with stroma consists of reticularfibers and cells.

5. Stroma offers a assistance network because that largenumbers of lymphocytes.

6. Node is composed of:

a. A dense outer cortex that is composed of numerouslymphatic nodules. Numerous lymphocytes, macrophages, various other antigen presentingcells (APCs), plasma cells and also reticulocytes are present. Follicular dendriticcells are uncovered in the germinal centers the lymph nodules that room in thecortex.

b. A less thick medulla consisting oflymphocytes i ordered it in strands referred to as medullary cords.

* sinuses are current in the medulla

* cords and sinuses expand toward a centralhilus the is essentially a huge trabecula projecting into the lymph node native connective tissue capsule.

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

** blood ship branch indigenous the hilus into thecortical region where they provide rise to \"bulbs\" that capillaries withinthe germinal centers of the cortical lymph nodules

7. Between the cortex and medulla is theparacortical an ar or thymic dependent zone the the node that includes denselypacked cells the are greatly T-lymphocytes.

a. This an ar lacks lymphocytes in animals thathave had actually the thymus eliminated at birth.

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


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

1. Palatine tonsils

a. On left and also right in behind area of dental 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 forms invaginationscalled many crypts thatpenetrate right into the tape of nodules.

d. These 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 separated fromunderlying organization by a partial capsule of thick connective tissue.


2. Pharyngeal tonsils

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

b. Greatly lie beneath a usual pseudostratifiedciliated columnar respiratory tract epithelium in rear roof the pharynx. Some locations of the spanning epithelium possibly stratified squamous.

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

3. Lingual tonsils

a. Situated in the root of tongue.

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

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

d. A slim partial capsule of dense connectivetissue the end the lymphoid tissue from basic tissue.

D. Thymus

1. Location - situated above the heart whereby thegreat ship connect.

2. Importance - throughout early life once the cellularmediated component of the immune system develops. Experience atrophy in laterlife, at which time it loses its sensible significance.

3. The thymus is composed of multiple lobes eachcontaining characteristic cortical and also medullary structure; however, this 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 tissues of embryo’s pharynx areinternalized during advance and move to website of thymus dorsal to heart.

b. These epithelial tissues are invaded bylymphoblasts (immature T-lymphocytes) that originate from stem cells in thebone marrow.

c. The invading cells organize themselves right into thecortical and medullary sections of lobules.

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

* this cells surround a central zone of looselymphatic tissue dubbed the medullary zone that is composed of fewer thymocytesand an ext epithelial reticular cells.

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

d. Various other cell varieties found in the thymusare:

* macrophages

* plasma cells

* mast cells

5. Cortical great of thymus

a. Site of lymphocyte manufacturing - departments oflymphoblast cells.

b. Thus, there is considerable mitotic activityof lymphoblasts

c. Epithelial reticular cell are much less numerousin this area and have thin and long processes that evelope groups of developingthymocytes. No reticular fibers are present.


d. This sheath of epithelial reticular cells, aswell as the desmosomes that connect surrounding reticular cells and the thickbasement membrane that underlies castle act to separate occurring thymocytesfrom the circulatory system.

6. Blood-thymus obstacle - only existing in thecortex, acts to prevent many blood born international antigens indigenous reachingdeveloping thymocytes. This obstacle consists of:

a. Non-fenestrated, consistent endothelium ofblood capillaries

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

c. Macrophages the are current in theconnective tissue.

d. Special basal lamina that epithelial reticularcells

e. Desmosome connections developing tight junctionsbetween adjacent endothelial cell of capillaries, as well as similarconnections between surrounding epithelial-reticular cells

7. Medullary zone

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

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

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

*sometimes they space calcified.

*these structures room characteristic that thymus.

8. Blood supply

a. Branches from the inner thoracic andinferior thyroid arteries that penetrate the capsule surrounding the thymus

b. Prolong into thymus along interlobular septa.

c. Capillaries branch right into the cortico-medullaryjunction area and also extend into the cortex. Blood-thymus barrier is present asdescribed above.

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

* In the medulla, the sheath cells of theblood-thymus barrier are lost and also vessels end up being permeable.

*cells might move right into or the end of the thymus inthis an ar by entering the circulatory system.

e. Venules affix to veins that exit thymusalong connective organization septa.

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

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

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

9. Physiology of thymus family member to immunesystem.

a. Recall the T-lymphocytes space responsible forcell-mediated immune responses and additionally for communicating with B-lymphocytes tocause those cell to develop antibodies against specific antigens.

b. The thymus can be thought about a proliferationand maturation center for T-lymphocytes. Precursor cells move from bonemarrow come thymus beginning the organ via blood ship in medullary zone. Thesecells undergo mitosis and maturation in cortical zone and then leave thymusthrough blood vessels of medullary zone to go about their assorted activities.This duty for the thymus is greatly completed shortly after puberty in ~ which timethe body organ undergoes “involution.” as this occurs, the thymus accumulation aconsiderable lot of unilocular fat. Tiny areas of energetic lymphoid tissueare retained and will proceed to produce little numbers of maturation T-lymphocytesfor the remainder the a person’s life; however, this task is reportedly not anecessity because the thymus of an adult deserve to be removed without resulting in immunesystem problems.

E. Spleen

1. Basic characteristics

a. Biggest piece the lymphatic tissue in body.

b. Site of development of caused lymphocytesthat go into the circulatory system.

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

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

2. Structure

a. Surrounded by a thick connective tissuecapsule the extends processes (trabeculae) right into lymphatic organization of thisorgan.

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

* A hilus that connective organization is presentmedially.

* Blood vessels and nerves run through the hilumand get in the spleenic pulp via the trabeculae. There are no lymph vessels inthe pulp.

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


b. Red pulp

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

* supported by a network the reticular cell andtheir associated fibers.

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

* Blood sinusoids are current - sites of cellularexchange between spleen and also circulatory system. Cells can go into or leaving spleenthrough huge spaces between endothelial cell lining sinusoids.

c. White pulp

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

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

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

*Marginal ar acts together a filter to pull foreignantigens the end of blood so the lymphocytes deserve to be set off to participate 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 cell areorganized into usual lymphatic nodules.

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


3. Blood circulation in spleen

a. Arteries get in pulp via trabeculae.

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

c. This arteries are surrounded by a sheath oflymphocytes greatly T-lymphocytes) that form the periarterial lymphatic sheath(PALS) extending along the size of the artery.

d. Branches of main artery extend into whitepulp.

e. Few of these leave white pulp and also then loopback toward it emptying right into sinusoids that form part of marginal zone the looselymphoid organization that surrounds white pulp.

f. Other branches of main arteries extendinto red pulp to form the pulp arteries that empty right into various sinusoids ofthis tissue.

g. Blood native the sinusoids is built up intored pulp veins. These trabecular veins combine to form the spleenic vein thatleaves the spleen with the hilus.

h. Blood circulation is most likely both open(blood empties into tissue that pulp the is percolates through and also is latercollected right into sinusoids that affix to venules) and also closed (blood indigenous arteriolesempties right into sinusoids that space directly connected to venules).

4. Attributes of the spleen

a. Manufacturing of blood cells

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

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

b. Devastation of erythrocytes

* Worn out erythrocytes are phagocytosed anddigested

* Hemoglobin is damaged down right into bilirubin andferritin.

c. Recycling the ferritin native worn outerythrocytes for synthesis that hemoglobin

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

e. Immune response

* website of activation that both T- andB-lymphocytes

* these cell species interact v dendritic cells(macrophages) that present antigen to these lymphocytes.

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f. Storage of erythrocytes that deserve to be releasedinto circulatory mechanism when needed.