Download Secondary lymphoid organs

Histology
Ass. Lec.
Dentistry College
Wafaa H. M. Alhashimy
Second Stage
Lab.-3
Lymphoid system
Lymphoid system includes all the cells , tissues and organs in the body
that contain aggregates of immune cells called lymphocytes .
The
lymphoid system performs numerous functions . it collects excess
interstial fluid into lymphatic capillaries , transports absorbed lipids from
the small intestine and responds immunologically to invading foreign .
The lymphoid system includes:
*Cells : lymphocytes (T-cells , B-cells ) , machrophage .
*lymphoid tissue
Non encapsulated aggregates of lymphocytes called lymphoid nodules
or follicles are found in loose connective tissue in digestive , respiratory
and reproductive systems .
*lymphoid organs
 Primary lymphoid organs:
Is
generate lymphocytes from immature
progenitor
cells.
The thymus and the bone marrow constitute the primary lymphoid tissues
involved in the production and early selection of lymphocytes.
 Secondary lymphoid organs:
are maintain mature lymphocytes and initiate an adaptive immune
response. The peripheral lymphoid organs are the sites of lymphocyte
activation by antigen. Secondary lymphoid tissue provides the
environment for the foreign or altered native molecules (antigens) to
interact with the lymphocytes. such as tonsils, spleen .
lymph nodes
The lymph node a loose connective tissue capsule with collagen fibers
surround the lymph node and sends its trabeculae into the interior . lymph
enters the lymph node via afferent lymphatic vessels that penetrate the
capsule on the convex surface .
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Each lymph nodes contains an outer cortex and an inner medulla . a
network of reticular fibers and spherical , non encapsulated aggregations
of lymphocytes called lymphoid nodules , characterize the cortex . some
lymphoid nodules exhibit lighter – staining central areas , called germinal
centers . the medulla consists of medullary cords and medullary sinuses ,
medullary cords are networks of reticular fibers that are filled with
plasma cells , macrophages and lymphocytes separated by capillary – like
channels called medullary sinuses . lymph flows through the medullary
sinuses and exits the lymph node on the opposite side via the efferent
lymphatic vessels .
Thymus gland
Is a soft to bulated lymphoepithelial organ located in upper anterior
mediastinum and lower part of the neck. The Thymus gland is surrounded
by connective tissue capsule .
cortex
under which is adark- staining cortex The cortical portion is mainly
composed of immature lymphoid cells, supported by a network of finelybranched epithelial reticular cells.
medulla
In lighter-staining medulla the lymphoid cells are relatively fewer in
number, and there are found peculiar nest-like bodies, the concentric
corpuscles of Hassall. These concentric corpuscles are composed of a
central mass, consisting of one or more granular cells, and of a capsule
formed of epithelioid cells. They are the remains of the epithelial tubes .
spleen
The spleen is a large lymphoid organ with a rich blood supply . a
connective tissue capsule surrounds the spleen and divides its interior
into incomplete compartments , called the splenic pulp . the spleen Like
the thymus that possesses only efferent lymphatic vessels
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White pulp
Consist of lymphatic nodules that surround a blood vessel called the
sentral artery .
Red pulp
which in turn consists of splenic cords and splenic sinusoids . splenic
cords contain networks of reticular fibers in which macrophages ,
lymphocytes , plasma cells and different blood cells are found . splenic
sinuses are interconnected blood channels that drain splenic blood into
larger sinuses that eventually leave the spleen via the splenic vein.
Tonsil
The paired palatine tonsils consist of aggregation of lymphatic nodules
located in the oral cavity and covered by stratified squamous non
keratinized eplium
lymph node
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Thymus gland
palatine tonsils
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