THYMUS

Thymus Thymus is the site of T-Cell differentiation and maturation it is a biolobed gland, situated above heart in the thorax region each lobe is encapsulated and it is dvided into lobules whiche are separated by strands of connective tissue called trabeculae Each lobule contain-lymphocytes& each organized into two compartments 1-Outer cortex 2- Inner medullaFunction:Site of T-cell maturation.Relative size greatest in newbornAbsolute size great at puberty

The cortex contains mostly immature and proliferating thymocytes,Medulla is sparly populated with thymocytes some of which mature and migrate to the medulla in medulla they learn to discriminate between self and non-self during fetal development and for a short time after birth. T-cell leave the medulla to enter the peripheral blood circulation, through which they are transported to the secondary lymphoid organs About 95% of all T cells die in thymus

Beside lymphoid cells it is composed of 1.Epithelial cells(cortical and medullary)2.Macrophage3.Dendritic cells4.Nurse cells5.Hassall,s corpusclesLymphocyte in the thymus are called thymocytes





Function of the thymus1-Generate and select T-cells2-Through clonal selection mechanism, thymus cause the death of those T-cell that cannot recognize Ag-MHC-complexes and those that react with self Ag-MHC and stop danger of causing autoimmune disease3- thus about 95% of all T cells die in the thymusNote:Thymectomy is surgical removal of thymusthymectomized mice show decrease in circulating lymphocyte and absence of cell mediated immunity

The size of the thymus varies with age

In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart
It increases in size and is most active during childhood
It stops growing during adolescence and then gradually atrophies

Lymph Nodes

Lymph nodes are the principal lymphoid organs of the body
Nodes are imbedded in connective tissue and clustered along lymphatic vessels
Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body
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Lymph Nodes

Their two basic functions are:
Filtration – macrophages destroy microorganisms and debris
Immune system activation – monitor for antigens and mount an attack against them
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Structure of a Lymph Node

Nodes are bean shaped and surrounded by a fibrous capsule
Trabeculae extended inward from the capsule and divide the node into compartments
Nodes have two histologically distinct regions: a cortex and a medulla
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Structure of a Lymph Node
The cortex contains follicles with germinal centers, heavy with dividing B cells
Dendritic cells nearly encapsulate the follicles
The deep cortex houses T cells in transit
T cells circulate continuously among the blood, lymph nodes, and lymphatic stream
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Structure of a Lymph Node

Medullary cords extend from the cortex and contain B cells, T cells, and plasma cells
Throughout the node are lymph sinuses crisscrossed by reticular fibers
Macrophages reside on these fibers and phagocytize foreign matter
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Structure of a Lymph Node

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Figure 20.4a, b

Circulation in the Lymph Nodes

Lymph enters via a number of afferent lymphatic vessels
It then enters a large subcapsular sinus and travels into a number of smaller sinuses
It meanders through these sinuses and exits the node at the hilus via efferent vessels
Because there are fewer efferent vessels, lymph stagnates somewhat in the node
This allows lymphocytes and macrophages time to carry out their protective functions
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Other Lymphoid Organs

The spleen, and tonsils
Peyer’s patches and bits of lymphatic tissue scattered in connective tissue
All are composed of reticular connective tissue and all help protect the body
Only lymph nodes filter lymph
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Spleen

Largest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm
It extends to curl around the anterior aspect of the stomach
It is served by the splenic artery and vein, which enter and exit at the hilus
Functions
Site of lymphocyte proliferation
Immune surveillance and response
Cleanses the blood by removing old RBC
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Additional Spleen Functions
Stores breakdown products of RBCs for later reuse
Spleen macrophages salvage and store iron for later use by bone marrow
Site of fetal erythrocyte production (normally ceases after birth)
Stores blood platelets
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Structure of the Spleen

Surrounded by a fibrous capsule, it has trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytes
Two distinct areas of the spleen are:
White pulp – area containing mostly lymphocytes suspended on reticular fibers and involved in immune functions
Red pulp – remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens
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Structure of the Spleen

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Figure 20.6a-d

Tonsils

Simplest lymphoid organs; form a ring of lymphatic tissue around the pharynx
Location of the tonsils
Palatine tonsils – either side of the posterior end of the oral cavity
Lingual tonsils – lie at the base of the tongue
Pharyngeal tonsil – posterior wall of the nasopharynx
Tubal tonsils – surround the openings of the auditory tubes into the pharynx
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Tonsils

Lymphoid tissue of tonsils contains follicles with germinal centers
Tonsil masses are not fully encapsulated
Epithelial tissue overlying tonsil masses invaginates, forming blind-ended crypts
Crypts trap and destroy bacteria and particulate matter
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Tonsilitis

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Peyer’s patches

Isolated clusters of lymphoid tissue, similar to tonsils
Found in the wall of the distal portion of the small intestine
Similar structures are found in the appendix
Peyer’s patches and the appendix:
Destroy bacteria, preventing them from breaching the intestinal wall
Generate “memory” lymphocytes for long-term immunity
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MALT

Mucosa-Associated Lymphatic Tissue is composed of:
Peyer’s patches, tonsils, and the appendix (digestive tract)
Lymphoid nodules in the walls of the bronchi (respiratory tract)
MALT protects the digestive and respiratory systems from foreign matter
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