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The Lymphatic System. Chapter 20. Introduction. The lymphatic system supports the function of the cardiovascular and immune systems of the body The lymphatic system consists of two semi-independent parts A network of lymphatic vessels Lymphoid organs scattered throughout the body.

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Introduction
Introduction

  • The lymphatic system supports the function of the cardiovascular and immune systems of the body

  • The lymphatic system consists of two semi-independent parts

    • A network of lymphatic vessels

    • Lymphoid organs scattered throughout the body


Introduction1
Introduction

  • The lymphatic vessels transport fluids that have escaped from the cardio-vascular system

  • The main components of the immune system (lymphocytes, lymphoid tissue, and lymphoid organs) fight infections and confer immunity to disease


The lymphatic system1
The Lymphatic System

  • An elaborate system of lymphatic vessels runs throughout the body

  • These vessels collect a fluid called lymph from the loose connective tissue around blood capillaries and carry this fluid to the great veins at the root of the neck


The lymphatic system2
The Lymphatic System

  • Because lymph flows only toward the heart, the lymphatic vessels form a one-way system rather than a full circuit


The lymphatic system3
The Lymphatic System

  • There are several orders of vessels

    • Lymph capillaries

    • Lymphatic collecting vessels

    • Lymph nodes

    • Lymph trunks

    • Lymph ducts


The lymphatic system4
The Lymphatic System

  • Recall that all blood capillaries are surrounded by a loose connective tissue that contains tissue fluid or interstitial fluid

  • The fluid arises from blood filtered through the capillary walls


The lymphatic system5
The Lymphatic System

  • Tissue fluid consists of small molecules of blood plasma, water, various ions, nutrient molecules, and respiratory gases


The lymphatic system6
The Lymphatic System

  • Tissue fluid is continuously leaving and re-entering the blood capillaries

  • For complex reasons slightly more fluid arises from the arteriole end than re-enters the venule end

  • This amounts to about 3 liters a day


The lymphatic system7
The Lymphatic System

  • The lymphatic vessels function to collect this excess fluid and return it to the bloodstream

  • Any blockage of the lymphatic vessels causes the affected body region to swell with excess tissue fluid resulting in edema


The lymphatic system8
The Lymphatic System

  • The lymphatic vessels also perform another related function

  • Blood proteins leak slowly from blood capillaries into the surrounding tissue fluid

  • Lymph vessels return leaked proteins to the bloodstream

  • This is important because proteins in blood generate osmotic forces that are essential for keeping water in the bloodstream


Lymph capillaries
Lymph Capillaries

  • Lymph capillaries are permeable vessels that receive the tissue fluid

  • They are located near blood vessels in the loose connective tissue

  • Like blood capillaries their walls consists of a single layer of endothelial cells


Lymph capillaries1
Lymph Capillaries

  • Lymph capillaries are very permeable

  • The permeability results from the structure and arrangement of the endothelial cells

  • They have few intercellular junctions


Lymph capillaries2
Lymph Capillaries

  • In lymph capillaries the adjacent edges of cells overlap to form easily opened mini- valves

  • These valves open to allow tissue fluid to enter the lymphatic capillary

  • Collagen filaments anchor the cells to the connective tissue


Lymph capillaries3
Lymph Capillaries

  • The minivalves work because the collagen filaments anchor portions of the cell wall to the surrounding connective tissue

  • Any fluid pressure in the volume of the tissue fluid separates the minivalves

  • This opens gaps in the wall of the capillary allowing fluid to enter

  • Once in the lymphatic capillary, it cannot leak out, because backflow pressure forces the minivalve flaps together


Distribution of lymphatic vessels
Distribution of Lymphatic Vessels

  • Lymph capillaries are widespread, occurring almost everywhere blood capillaries occur

  • Lymph capillaries are absent from bone and teeth, bone marrow, and the entire central nervous system


Lymph capillaries4
Lymph Capillaries

  • Once interstitial fluid enters the lymphatic ducts it is called lymph

  • Although the high permeability of lymph capillaries allows the uptake of large quantities of tissue fluid and protein molecules it also allows bacteria, viruses, or cancer cells in the loose connective tissue to enter these capillaries with ease


Lymph capillaries5
Lymph Capillaries

  • These pathogenic agents can then travel throughout the body via the lymphatic vessels

  • However, most pathogenic agents are destroyed in the lymph nodes by various antibodies before reaching the general circulation

  • Cancer cells can be the most problematic and can actually use lymph nodes as a site to metastasize further


Lymph capillaries6
Lymph Capillaries

  • Lymph capillaries are widespread, occurring almost everywhere blood capillaries occur


Lymph capillaries7
Lymph Capillaries

  • Lymph capillaries are absent from bone and teeth, from bone marrow, and from the entire CNS

  • In the CNS excess fluid drains into the cerebrospinal fluid and then returns it to the blood at the superior sagittal sinus


Lymph collecting vessels
Lymph Collecting Vessels

  • From the lymph capillaries, lymph enters lymphatic collecting vessels which accompany blood vessels


Lymph collecting vessels1
Lymph Collecting Vessels

  • In general, the superficial lymphatic collecting vessels in the skin travel with superficial veins

  • Deep lymphatic collecting vessels of the trunk and digestive viscera travel with the deep arteries


Lymph collecting vessels2
Lymph Collecting Vessels

  • Lymphatic collecting vessels are narrow and delicate and are usually not seen in a laboratory dissection

  • The vessels have the same tunics as blood vessels but their walls are always thinner

  • The thinness reflects the fact that lymph flows under very low pressure and have no pump to move the lymph along


Lymph collecting vessels3
Lymph Collecting Vessels

  • To direct the flow of lymph, lymphatic collecting vessels contain more valves than do veins

  • At the base of each valve, the vessel bulges, forming a pocket in which lymph collects and forces the valve shut


Lymph collecting vessels4
Lymph Collecting Vessels

  • Because of these bulges, each collecting vessel resembles a string of beads

  • This distinctive appearance, which characterizes the larger lymph ducts as well allow MD’s to recognize lymph vessels in X-rays


Lymph collecting vessels5
Lymph Collecting Vessels

  • Unaided by pressure from a pump, lymph is propelled through lymph vessels by a series of weaker mechanisms

  • The action of contracting skeletal muscle and the pulsation of nearby arteries push on lymph vessel, squeezing lymph through them


Lymph collecting vessels6
Lymph Collecting Vessels

  • The muscular tunica media of the lymph vessels also contacts to help propel the lymph

  • Additionally, the normal movements of the limbs and trunk keep the lymph flowing

  • Despite these mechanisms, the transport of lymph is slow

  • People who are inactive or who stand for long times often develop edema


Lymph nodes
Lymph Nodes

  • Lymph nodes, which cleanse the lymph of pathogens are bean shaped organs situated along lymphatic collecting vessels

  • The term lymph gland is not correct because they are not glandular in their function

  • There are about 500 lymph gland in the body


Lymph nodes1
Lymph Nodes

  • Large clusters of superficial lymph nodes in the cervical, axillary, and inguinal regions

  • The superficial cervical nodes along the jugular and carotid arteries receive lymph from the head and neck


Lymph nodes2
Lymph Nodes

  • Axillary nodes in the armpit filter lymph from the upper limbs


Lymph nodes3
Lymph Nodes

  • The inguinal nodes in the superior thigh filter lymph from the lower limb


Lymph nodes4
Lymph Nodes

  • Nodes in the mediastinum such as the deep tracheobronchial nodes receive lymph from the thoracic viscera


Lymph nodes5
Lymph Nodes

  • Deep nodes along the abdominal aorta, called aortic nodes, filter lymph from the posterior abdominal wall


Lymph nodes6
Lymph Nodes

  • Finally, deep nodes along the iliac arteries, called iliac nodes, filter lymph from the pelvic organs and the lower limbs


Lymph nodes7
Lymph Nodes

  • The microscopic anatomy of a lymph node suggests its role as a body filter

  • The node is surrounded by a fibrous capsule of dense connective tissue


Lymph nodes8
Lymph Nodes

  • Fibrous strands of connective tissue called trabecule extend inward to divide the node into compartments


Lymph nodes9
Lymph Nodes

  • Lymph enters the convex aspect of the node through several afferent lymphatic vessels and exits from the indented region on the other side, the hilus, through efferent lymphatic vessels


Lymph nodes10
Lymph Nodes

  • Between the afferent and efferent vessels, lymph percolates through lymph sinuses

  • These large lymph capillaries are spanned internally by a crisscrossing network of reticular fibers


Lymph nodes11
Lymph Nodes

  • The reticular fibers are covered by star-shaped endothelial cells

  • Many macrophages live on this fiber network, phagocytizing pathogens and foreign particles in the lymph that flows through the sinuses


Lymph nodes12
Lymph Nodes

  • Most lymph passes through several nodes

  • It is usually free of pathogens by the time it leaves its last node and enters the lymph trunks on its way to the great veins in the neck


Lymph nodes13
Lymph Nodes

  • Along with its lymph sinuses, a lymph node also contains tadpole shaped masses of lymphoid tissue

  • This tissue is divided into outer (cortex) and inner (medulla) regions and are part of the immune system


Lymph trunks
Lymph Trunks

  • After leaving the lymph nodes, the largest lymphatic collecting vessels converge to form lymph trunks

  • These trunks drain large areas of the body


Lumbar trunks
Lumbar Trunks

  • The paired lumbar trunks lie along either side of the aorta in the inferior abdomen

  • They receive all lymph draining from the lower limbs, pelvic organs and some of the anterior abdominal wall


Intestinal trunk
Intestinal Trunk

  • The unpaired intestinal trunk lies near midline on the posterior abdominal wall

  • It receives fatty lymph (chyme) from the stomach, intestines, and other digestive organs


Brachiomediastinal trunks
Brachiomediastinal Trunks

  • These paired trunks ascend near the sides of the trachea

  • They collect lymph from the thoracic viscera and thoracic wall


Subclavian trunks
Subclavian Trunks

  • These are paired trunks located near the sides of the trachea

  • These trunks receive lymph from the upper limbs

  • They also drain the inferior neck and the superior thoracic wall


Jugular trunks
Jugular Trunks

  • These trunks are located in the neck at the base of each internal jugular vein

  • These trunks drain lymph from the head and neck


Lymph ducts
Lymph Ducts

  • The lymph trunks drain into the largest vessels the lymph ducts

  • The number of ducts in an individual may vary from two to only only duct


Thoracic duct
Thoracic Duct

  • The thoracic duct is present in all individuals

  • Its most inferior part, located at the union of the lumbar and intestinal trucks is the cisterna chyli which lies on the bodies of vertebrae L1 + L2


Thoracic duct1
Thoracic Duct

  • The thoracic duct ascends along the vertebral bodies

  • In the superior thorax, it turns left and empties into the venous circulation at the junction of the internal jugular and left subclavian veins


Right lymphatic duct
Right lymphatic Duct

  • Some people have a short right lymphatic duct formed by the union of the right jugular, subclavian, and broncho-mediastinal trunks


Right lymphatic duct1
Right lymphatic Duct

  • When present, this duct empties into the neck veins at or near the junction of the right internal jugular and subclavian veins

  • The right lymphatic duct drains the upper quarter of the body


Right lymphatic duct2
Right lymphatic Duct

  • In individuals without a right lymphatic duct, the three trunks empty separately into the neck veins


Lymphatic vessels
Lymphatic Vessels

  • In summary, the lymphatic vessels….

    • Return excess tissue fluid to the bloodstream

    • Return leaked proteins to the blood

    • Carry absorbed fat from the intestine to the blood through lacteals


Immune system
Immune System

  • The immune system is central to the body’s fight against disease

  • Unlike the body’s other defense systems, it recognizes and attacks specific foreign molecules

  • It destroys pathogens more and more effectively with each new exposure


Immune system1
Immune System

  • The immune system centers around the key defense cells from lymphocytes

  • But it also includes lymphoid tissue, and the lymphoid organs these include…

    • Lymph nodes, spleen, thymus, tonsils, aggregated lymphoid nodules in the small intestine, and appendix


Lymphocytes
Lymphocytes

  • Infectious microorganisms that penetrate the epithelial barriers of the body enter the underlying loose connective tissues, where they are attacked by the inflammatory response, by macrophages and finally, by lymphocytes of the immune system

  • Lymphocytes are white blood cells and that each lymphocyte recognizes and attacks its own type of foreign molecule, called an antigen


Lymphocytes1
Lymphocytes

  • B lymphocytes multiply to become plasma cells that secrete antibodies

  • Cytotoxic (CD8+) T lymphocytes destroy antigen bearing cells by penetrating their membranes and inducing programmed cell death


Lymphocytes2
Lymphocytes

  • B and T cells continuously travel in the blood and lymph streams to reach infected connective tissues throughout the body, where they fight infection

  • They repeatedly enter and exit these connective tissues, including the often infected lymphoid tissue, by squeezing through the walls of capillaries and venules


Lymphocytes3
Lymphocytes

  • This repeated movement of activated lymphocytes between the circulatory vessels and the connective tissues, called recirculation, ensures that lymphocytes reach all infection sites quickly


Lymphocyte activation
Lymphocyte Activation

  • Immature lymphocytes go through several stages before they are able to attack antigens

  • Most lymphocytes pass through these stages during infancy and childhood, by many do so in adulthood as well


Lymphocyte activation1
Lymphocyte Activation

  • This illustration provides an overview of lymphocyte activation


Lymphocyte activation2
Lymphocyte Activation

  • Lymphocytes originate in the bone marrow from lymphoid stem cells, some of which travel in the bloodstream to the thymus in the thorax and become T lymphocytes (T is for Thymus)


Lymphocyte activation3
Lymphocyte Activation

  • Other lymphocytes stay in the bone marrow and become B lymphcytes


Lymphocyte activation4
Lymphocyte Activation

  • These new T and B lymphocytes divide rapidly and generate many lymphocyte families (clones), each of which is able to recognize one unique type of antigen (this is called gaining immunocompetence)


Lymphocyte activation5
Lymphocyte Activation

  • Young T or B lymphocytes travel through the bloodstream to an infected connective tissue, where it binds to its specific antigen, an encounter called antigen challenge


Lymphocyte activation6
Lymphocyte Activation

  • As a consequence of the antigen challenge the lymphocyte becomes fully activated, gaining the ability to attack its antigen, proliferates rapidly, and produces mature lymphocytes that recirculate throughout the body seeking pathogens to attack


Lymphocyte activation7
Lymphocyte Activation

  • During the antigen challenge, an activating lymphocyte interacts with several other cells types

  • The lymphocyte receives its antigen from an antigen presenting cell, such as a macrophage that has recently phagocytized the antigen, or a star shaped dendritic cell, a professional antigen gathered that patrols the body seeking antigens and carries them to places where lymphocytes gather


Lymphocyte activation8
Lymphocyte Activation

  • A distinct type of lymphocyte, called a helper (CD4+) T lymphocyte, secretes chemical signals that greatly stimulate the proliferation of activating B and cytotoxic T lymphocytes

  • Helper T cells are important because their signals amplify and fine-tune the immune response


Lymphocyte activation9
Lymphocyte Activation

  • The importance of helper T lymphocytes is illustrated by acquired immune deficiency syndrome (AIDS), a viral disease in which a drastic decline in the body’s helper T cells greatly weakens the immune system


Lymphocyte activation10
Lymphocyte Activation

  • As more activating T or B cells proliferates within infected connective tissue, it produces two types of mature lymphocytes, effector and memory

  • Short lived effector lymphocytes attack the pathogen immediately and then die

  • Memory lymphocytes, by contrast, wait until the body encounters their antigen again - maybe decades later


Lymphocyte activation11
Lymphocyte Activation

  • When a memory lymphocyte finally encounters its antigen, its proliferating response and its attack are most vigorous and rapid

  • Memory lymphocytes are the basis for acquired immunity

  • They guard against subsequent infections and prevent us from getting many diseases more than once

  • There are T and B varieties of memory lymphocytes


Lymph nodes clinical
Lymph Nodes: Clinical

  • Inflammation of a node is caused by a large number of bacteria trapped in a node

    • Inflammation results in swelling and pain

  • Lymph nodes can become secondary cancer sites, particularly in metastasizing cancers that enter lymphatic vessels and become trapped

    • Cancer infiltrated nodes are swollen but not painful


Lymphoid tissue
Lymphoid Tissue

  • Lymphoid tissue is an important component of the immune system because it

    • Houses and provides a proliferation site for lymphocytes

    • Furnishes an ideal surveillance vantage point for both lymphocytes and macrophages


Lymphoid tissue1
Lymphoid Tissue

  • Lymphoid tissue is the most important tissue of the immune system

  • The tissue is an often infected connective tissue in which vast quantities of lymphocytes gather to fight invading microorganisms

  • This tissue had two general locations

    • Mucous membranes

    • Lymphoid organs


Lymphoid tissue2
Lymphoid Tissue

  • Mucous membranes

    • Found within the digestive, respiratory, urinary and reproductive tracks where it is called mucosa-assocaited lymphoid tissue (MALT)

  • Lymphoid Organs

    • Lymph nodes, spleen, thymus, tonsils, aggregated lymphoid nodules, and appendix


Lymphoid tissue3
Lymphoid Tissue

  • This tissue is the main battleground in the fight against infection

  • Lymphoid tissue is where most lymphocytes become activated and most effector and memory lymphocytes are generated


Lymphoid tissue4
Lymphoid Tissue

  • Lymphoid tissue, a type of loose connective tissue called reticular connective tissue, dominates all lymphoid organs except the thymus

  • The dark staining areas represent the connective tissue fibers


Lymphoid tissue5
Lymphoid Tissue

  • The structural features of lymphoid tissue serves its infection fighting role

  • It is a reticular connective tissue whose basic framework is a network of reticular fibers secreted by reticular cells (fibroblasts)


Lymphoid tissue6
Lymphoid Tissue

  • Within the spaces of this network reside the many T and B lymphocytes that arrive continuously from venules coursing through the tissue

Macrophage

Lymphocytes

Reticular

fiber


Lymphoid tissue7
Lymphoid Tissue

  • Macrophages on the fiber network kill invading microorganisms by phagocytosis and along with dendritic cells, they activate nearby lymphocytes by presenting them with antigens

Macrophage

Lymphocytes

Reticular

fiber


Lymphoid tissue8
Lymphoid Tissue

  • Evident within lymphoid tissues are scattered, spherical clusters of densely packed lymphocytes, called lymphoid nodules or follicles

  • These nodules often exhibit lighter staining germinal centers


Lymphoid tissue9
Lymphoid Tissue

  • Nodules derive from the activation of a single B cell, whose rapid proliferation generates the thousands of lymphocytes in the nodule

  • Newly produced B cells migrate away from the nodule to become plasma cells


Lymphoid organs
Lymphoid Organs

  • Lymphoid organs are the lymph nodes, spleen, thymus, aggregated lymphoid nodules in the small intestine, and appendix


Lymphoid organs1
Lymphoid Organs

  • Lymph nodes are more than filters

  • The regions of the node between the lymph sinuses are tadpole shaped masses of lymphoid tissue


Lymphoid organs2
Lymphoid Organs

  • As lymph moves through the sinuses, some of the contained antigens leak out through the sinus wall into the lymphatic tissue

  • Most antigen challenges in the human body occur in the lymph nodes


Lymphoid organs3
Lymphoid Organs

  • In the lymph nodes antigens are destroyed and B and T lymphocytes are activated

  • The activation adds to the body’s supply of memory lymphocytes that offer long term immunity


Lymphoid organs4
Lymphoid Organs

  • Lymph nodes have two histologically distinct regions, an external cortex and a medulla

  • All the lymphoid nodules and most B cells occupy the lymphoid tissue of the most superficial part of the cortex


Lymphoid organs5
Lymphoid Organs

  • Deeper in the cortex the lymphocytes are primarily T cells, especially helper T cells that increase the activity of B cells in the nearby nodules


Lymphoid organs6
Lymphoid Organs

  • Thin, inward extensions from the cortical lymphoid tissue help define the medulla

  • These cord like medullary extensions contain both T and B lymphocytes, plus plasma cells


Spleen
Spleen

  • The soft, blood rich spleen is the largest lymphoid organ

  • Its size varies greatly among individual, but on average it is the size of a fist


Lymph nodes14
Lymph Nodes

  • Dendritic cells nearly encapsulate the follicles and abut the rest of the cortex, which primarily houses T cells in transit

  • The T cells circulate continuously between the blood, lymph nodes, and lymphatic stream, performing their surveillance role


Spleen1
Spleen

  • The large splenic vessels enter and exit the spleen on the anterior surface along a line called the hilus


Spleen2
Spleen

  • The spleen has two main blood cleansing functions

    • The removal of blood-borne antigens (its immune function)

    • The removal and destruction of aged or defective blood cells

  • Additionally, the spleen is a site of hematopoiesis in the fetus and stores blood platelets throughout life


Spleen3
Spleen

  • The spleen is surrounded by a fibrous capsule from which trabeculae extend inward

  • The larger branches of the splenic artery run in the trabeculae and send smaller arterial branches into the substance of the spleen


Spleen4
Spleen

  • The arterial branches are called central arteries because they are enclosed by thick sleeves of lymphoid tissue that collectively constitute the white pulp of the spleen


Spleen5
Spleen

  • Blood borne antigens enter this lymphoid tissue and are destroyed as they activate the immune response

  • Surrounding the white pulp is red pulp which has two parts

    • Venous sinuses

    • Splenic cords


Spleen6
Spleen

  • Venous sinuses are sinusoid capillaries that arise from the distal branches of the central arteries outside of the white pulp


Spleen7
Spleen

  • Splenic cords consist of reticular connective tissue that is exceptionally rich in macrophages

  • Whole blood leaks from the sinuses into this connective tissue where macrophages then phagocytize any defective blood cells


Spleen8
Spleen

  • Red pulp is responsible for the spleen’s ability to dispose of worn-out blood cells

  • White pulp provides the immune function of the spleen


Thymus
Thymus

  • The two lobed thymus lies in the anterior thorax and inferior neck, just posterior to the sternum


Thymus1
Thymus

  • The thymus is the site at which immature lymphocytes develop into T lymphocytes

  • The thymus secrets thymic hormones such as thymopoietin which causes T lymphocytes to gain immunocompetence


Thymus2
Thymus

  • Prominent in newborns, the thymus continues to increase in size during childhood when it is most active

  • During late adolescence, it begins to atrophy gradually, as its functional tissue is slowly replaced with fibrous and fatty tissue


Thymus3
Thymus

  • At age 20 it still has about 80% of its functional tissue but at age 40 it typically retains only 5% of its functional tissue

  • By age only 2% of functional tissue remains and the thymus is a fatty mass that is difficult to distinguish from surrounding connective tissue

  • However, even as it atrophies, the thymus continues to produce immunocompetent cells throughout adulthood (reduced rate)


Thymus4
Thymus

  • Again, the thymus lies in the superior thorax


Thymus5
Thymus

  • The thymus contains numerous lobules arranged like fronds in the head of a cauliflower

  • Each lobule contains an outer cortex and an inner medulla


Thymus6
Thymus

  • The cortex is packed with rapidly dividing T lymphocytes gaining immunocompetence

  • The medulla contains fewer lymphocytes

  • In addition, the medulla contain the thymic (Hassall’s) corpuscles which seem to be collections of degenerating epithelial reticular cells

  • The number and size of these corpuscles increases with age


Thymus7
Thymus

  • The thymus differs from other lymphoid organs in two basic ways (First)

    • It functions strictly in lymphocyte maturation and thus is the only lymphoid organ that does not directly fight antigens

    • The blood-thymus barrier, keeps blood-borne antigens from leaking out of thymic capillaries and prematurely activating the immature thymic lymphocytes


Thymus8
Thymus

  • The thymus differs from other lymphoid organs in two basic ways (Second)

    • The tissue framework of the thymus is not a true lymphoid connective tissue

    • The thymus arises like a gland from the epithelium lining the embryonic pharynx

    • Its basic tissue framework consists of star shaped epithelial cells rather that reticular fibers


Thymus9
Thymus

  • The thymus differs from other lymphoid organs in two basic ways (Second)

    • These epithelial reticular cells secrete the thymic hormones that stimulate T cells to become immunocompetent

    • The thymus has no lymphoid nodules because it lack B cells


The tonsils
The Tonsils

  • The tonsils are perhaps the simplest lymphoid tissue

  • They are mere swellings of the mucosal lining of the pharynx


The tonsils1
The Tonsils

  • There are four groups of tonsils, palatine, lingual, pharyngeal, and tubal


The tonsils2
The Tonsils

  • The palatine tonsils lie directly posterior to the mouth and palate on the lateral sides of the pharyngeal wall

  • These are the largest tonsils and the ones most often infected and removed during childhood (tonsillectomy)

  • The lingual tonsil lie on the posterior surface of the tongue


The tonsils3
The Tonsils

  • The pharyngeal tonsil (adenoids) lie on the pharyngeal roof

  • The tubal tonsils are just behind the openings of the pharyngotympanic tubes into the pharynx


The tonsils4
The Tonsils

  • The four tonsils are arranged in a ring around the entrance to the pharynx to gather and remove many pathogens that enter the pharynx in inspired air and swallowed food

  • The tonsils process the antigens, then set up immune responses


The tonsils5
The Tonsils

  • The tonsils consist of an epithelium underlain by a connective tissue lamina propria


The tonsils6
The Tonsils

  • In the tonsils the underlying propria consists of abundant mucosa associated lymphoid tissue (MALT) packed with lymphocytes and scattered lymphoid nodules

Germinal

centers


The tonsils7
The Tonsils

  • The overlying epithelium invaginates deep into the interior forming blind ended structures called crypts that trap bacteria and particulate matter


The tonsils8
The Tonsils

  • The trapped bacteria work their way through the epithelium into the underlying lymphoid tissue causing the activation of lymphocytes


The tonsils9
The Tonsils

  • The trapping of bacteria in the crypts leads to many infections during childhood

  • But it also generates a great variety of memory lymphocytes for long-term immunity


The tonsils10
The Tonsils

  • By inviting an infection, the tissue produces a wide variety of immune cells with a “memory” for the trapped pathogens

  • The early risk during childhood results in better health in adulthood


Aggregates of lymphoid follicles
Aggregates of Lymphoid Follicles

  • Many bacteria permanently inhabit the hollow interior of the intestines and are constantly infecting the intestinal walls

  • To fight these invaders, MALT is especially abundant in the intestine

  • In two parts of the intestine, MALT is so large, permanent, and densely packed with lymphocytes that is said to form lymphoid organs: Aggregated lymphoid nodules and the appendix


Aggregates of lymphoid follicles1
Aggregates of Lymphoid Follicles

  • Aggregated lymphoid nodules (Peyer’s patches) are clusters in the walls of the distal part of the ileum of the small intestine

  • About 40 of these patches are present averaging about 1 cm and 1 cm wide

Peyer’s Patches


Aggregates of lymphoid follicles2
Aggregates of Lymphoid Follicles

  • Aggregates of lymphoid follicles are ideally situated to destroy bacteria thereby preventing these pathogens from breaching the intestinal wall


The appendix
The Appendix

  • The appendix is a tubular offshoot of the first part of the (cecum) of the large intestine


The appendix1
The Appendix

  • Lymphoid tissue is also heavily concentrated on the walls of the appendix

  • Histological sections reveal that dense lymphoid tissue uniformly occupies over half the thickness of the wall of the appendix


The appendix2
The Appendix

  • Beside destroying the microorganisms that invade them, the aggregated lymphoid nodules and the appendix sample many different antigens from within the digestive tube and generate a wide variety of memory lymphocytes to protect the body


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