Distribution of Lymphoid Nodules
Lymph nodes
Spleen
Respiratory tract (tonsils)
Along digestive and urinary tracts
Mucosa-Associated Lymphoid Tissue (MALT)
Lymphoid tissues associated with the digestive system
Aggregated lymphoid nodules
Clustered deep to intestinal epithelial lining
Appendix (or vermiform appendix)
Contains a mass of fused lymphoid nodules
The Five Tonsils
In wall of pharynx
Left and right palatine tonsils
Pharyngeal tonsil (adenoid)
Two lingual tonsils
Lymphoid Organs
Lymph nodes
Thymus
Spleen
Are separated from surrounding tissues by a fibrous connective tissue capsule
Lymph Nodes
Trabeculae
Hilum
Trabeculae
Bundles of collagen fibers
Extend from capsule into interior of lymph node
Hilum
A shallow indentation where blood vessels and nerves reach the lymph node
Afferent lymphatic vessels
Carry lymph:
from peripheral tissues to lymph node
Efferent lymphatic vessels
Leave lymph node at hilum
Carry lymph to venous circulation
Lymph from Afferent Lymphatics
Flows through lymph node in a network of sinuses
lymph flow through a network of sinuses
From subcapsular space: contains macrophages and dendritic cells
Through outer cortex: contains B cells within germinal centers
Through deep cortex: dominated by T cells
Through the core (medulla): contains B cells and plasma cells, organized into medulla
Lymph Node
A filter
-Purifies lymph before return to venous circulation
Removes
-Debris
-Pathogens
-99% of antigens
Antigen Presentation
First step in immune response
Extracted antigens are "presented" to lymphocytes
-Or attached to dendritic cells to stimulate lymphocytes
Lymphoid Functions-
Lymphoid tissues and lymph nodes
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Lymph Nodes of Gut, Trachea, Lungs, and Thoracic Duct
Protect against pathogens in digestive and respiratory systems
Lymph Nodes (Glands)
Large lymph nodes at groin and base of neck
Swell in response to inflammation
Lymphadenopathy
Chronic or excessive enlargement of lymph nodes may indicate infections, endocrine disorders, or cancer
The Thymus
Located in mediastinum
Atrophies after puberty
Diminishing effectiveness of immune system
Divisions of the Thymus
Thymus is divided into two thymic lobes
Septa divide lobes into smaller lobules
A Thymic Lobule
Contains a dense outer cortex and a pale central medulla
Lymphocytes
Divide in the cortex
T cells migrate into medulla
Mature T cells leave thymus by medullary blood vessels
Reticular Epithelial Cells in the Cortex
Surround lymphocytes in cortex
Maintain blood-thymus barrier
-Secrete thymic hormones that stimulate
-Stem cell divisions
-T cell differentiation
Reticular Epithelial Cells in the Medulla
Form concentric layers known as thymic (Hassall) corpuscles
The medulla has no blood-thymus barrier
T cells can enter or leave bloodstream
Thymus Hormones
Thymosin, an extract from the thymus that promotes development of lymphocytes
Three Functions of the Spleen
1-Removal of abnormal blood cells and other blood components by phagocytosis
2-Storage of iron recycled from red blood cells
3-Initiation of immune responses by B cells and T cells
In response to antigens in circulating blood
Structure of the Spleen
Attached to stomach by gastrosplenic ligament
Contacts diaphragm and left kidney
Splenic veins, arteries, and lymphatic vessels
-Communicate with spleen at hilum
Structure of the Spleen Inside fibrous capsule
Red pulp: contains many red blood cells
White pulp: resembles lymphoid nodules
Trabecular Arteries
Branch and radiate toward capsule
Finer branches surrounded by white pulp
Capillaries discharge red blood cells into red pulp
Red Pulp
Contains elements of circulating blood
Plus fixed and free macrophages
Splenic Circulation
Blood passes through
Network of reticular fibers
Then enters large sinusoids (lined by macrophages)
Which empty into trabecular veins
Phagocytes and other lymphocytes in spleen
Identify and attack damaged and infected cells
In circulating blood